Mitsubishi G4M (1939)

Twin engine Naval Bomber 1939-1945: 2,435 Made.

The Mitsubishi G4M (allied conename "Betty") was the replacement for the 1930s G3M ("Nell"), as the primary land based bomber of the Imperial Japanese Navy. The Mitsubishi Navy Type 1 attack bomber nicknamed "Hamaki" as it was far more "fattier" than the nimble G3M, was also far more powerful and it boasted good performance across the board. It was considered the world's best land-based naval bomber, in a category that pretty much only existed in Japan. However this model, due to the obstination of the IJNAF command to sacrifice everything for range, had a lightweight structure and wings acting as fuel tanks.

It was later criticized for its lack of protection. Introduced on 2 April 1941, it was soon derided by allied pilots as the "The Flying Lighter", prone to ignition after just a hit. Only with the late G4M2 these issues were partly solved. With 2,435 delivered until 1945, it was also the most common Japanese twin engine torpedo bomber of WW2, with a range rivalling 4-engine bombers and took part in all important campaigns.

⚠ Note: Since this Puppy was arguably the most important Japanese bomber of WW2, it will be split into two parts as there is a lot to talk about. Part II to come with the next Japanese update.

The Prime IJN Land-Based Bomber

The Mitsubishi G4M was designed in 1937-38 already as a replacement for the earlier G3M, already an important asset for Yamamoto in its Pacfc grand strategy, giving the fleet a reach equivalent to aircraft carriers, but from captured airbases dotted across islands. At the time, the IJNAS, the air service branch of the Imperial Japanese Navy was the only one across the globe to feature large numbers of land-based bombers. That category was indeed as much as forbidden by law in the US (reserved for the USAAC) and in Britain, the Fleet Air Arm was only authorized seaplane bombers instead. But the rivalry between the Army and Navy resulted in this unique case of entirely separated air service branches. The G3M developed in 1933-34 was already largely inspired by Junkers airplanes, which patents were acquired from 1928, and this culminated with the very aerodynamic Mitsubishi Ka-9. The core idea was to design a bomber so fast it did not needed an escort.

Production of the G3M started via the Ka-15 and pre-series, really starting in 1936, then going on with improvements until 1943, with transport versions developed on that base. However by 1942 standards it was already considered too slow and undepowered. Its replacement was planned long in advance by Yamamoto, which wanted a faster and more powerful model, which would be still a twin-engine, land-based medium bomber. The Mitsubishi Aircraft Company was not firmly chosen at first, but a string contender to deliver this successor for the Air Service (IJNAS) with a scheduled debut date FY1940. The base design of the G3M was improved across the board, and it was fitted with engines almost twice as powerful. The end product, that first flew in 1939, became the Mitsubishi Navy Type 1 attack bomber (Ichishiki rikujō kōgeki ki, Isshikirikukō). But in service the G4M1 was soon hated or derided by lany IJN pilots when compared to the more graceful G3M. It was nicknamed "Hamaki" ("cigar") between its barrel-like fuselage shape and tendency to ignite if hit. As soon as it was spotted, the Allied reported its name as "Betty". It soon became a familiar codename in operaton as the G4M was virtually everywhere during the war.

The G4M was indeed designed to succeed the Mitsubishi G3M and concentrate better performances overall, excellent range and better payload. It was soon considered the best land-based naval bomber, boasting an agility that would leave 1941 allied fighters like the Buffalo and Wildcat, in the dust. The reality was severe however. Compromises made in its flimsy structure (like for the Zero) led to an almost total lack of crew protection, no armor plating, not even self-sealing fuel tanks, which became the norm in 1937-38 already. The G4M took its sweet time in development until finally declared ready for service and accepted by the IJNAS on 2 April 1941. If lightweight made sense for a dogfighter like the Zero, this was a death sentence of a bomber, as fighters would always catch up. The result was heavy losses, so much so, Allied fighter nicknamed the Betty "The Flying Lighter". It took a more year for Mitsubushi to start better protecting it, with the G4M2 and G4M3 (self-sealing fuel tanks, armor protection, better defensive armament), compensating for the extra weight with better engines.

Despite its youth issues, the G4M soon became the IJNAS' primary land-based bomber, bringing an unrivalled bombing capacity at long distances and another asset in the IJN arsenal, complementary alongside the Kido Butai (IJN carrier group). This became most widely produced and most famous Japanese bomber in world War II, reflecting the same path a the A6M in its field. The G4M took part in virtually all battles of the Pacific War, taking part for example to the sinking of Force Z (HMS Prince of Wales and Repulse) a world's first in the open sea. G4Ms would operate alongside G3Ms there and later to sink USS Chicago at the Battle of Rennell Island. As the G4M replacement was already on its way, between the 4-engined G6M "Renzan" and nimbler Yokosuka P1Y "Ginga" more in tone with the previous G3M, the last G4M took part in desperate conversions like a mother plane for the Yokosuka MXY-7 Ohka suicide craft. There was a 1940 heavy fighter derivative, the Mitsubishi G6M1 usable as long-range escort fighter but later converted as a transport. No fully intact G4M survived the war but several airframes were saved from the elements and remained in unrestored state or partial restoration.

Development of the Mitsubishi G4M

The very origin was the emergence of a type of aircraft known as the "rikujo kogeki-ki" or "basic attack aircraft" (abbreviated "Rikko"), had its own drive in 1932 Shōshō (Rear Admiral) Isoroku Yamamoto's own will to develop that model, as future commander of the Combined Fleet. He was at the head of the Technical Department of the Naval Bureau of Aeronautics (Kaigun Koku Hombu) and initiated a three-year plan to achieve tech. parity with The West on naval aviation. In IJNAF terminology, the "naval attack aircraft" (kogeki-ki) emerged. Its primary function was to attack enemy ships with torpedoes while being also capable of carrying out bombing strikes, unlike the "pure" bomber or "bakugeki-ki". This model was the brainchild of Shigeru Marsuyama, head of the Kaigun Koku Hombu, naval attack aircraft type. He imagined a multi-seat, multi-engine aircraft having the range to strike any navala asset with torpedoes or bombs, far from home Islands and deployed from forward airfields but also capable of supporting the Battle Fleet from the air.

Marsuyama ordered Yamamoto and his subordinates in the technical department to conduct research and development in this direction, which led to the 1934 9-shi specification, issued to the Mitsubishi Aircraft Company. This was the first aircraft of this new class, the Type 96 or G3M "96 Rikko". It was accepted into service in 1936 G3M (later the "Nell"). This all-metal, modern and very aerodynamic twin-engine aircraft with smooth skin and retractable landing gear was for the time, grounbreaking. As there was no naval bomber to compare, this really was a peculiar asset for the IJN. The G3M1 Model 11 could carry an 800 kg (1,750 lb) payload over 2,800 km (1,540 nautical miles) when the German Heinkel He 111B carried twice that load, but on 1,100 km. From April 1936, shortly after the formation of the Kisarazu Kokutai, first naval bomber unit with the G3M, Taisa (Captain 2nd Rank) Takijiro Onishi, as the head of the instructor department at Kaigun Koku Hombu, inspected the newly formed unit. Commander Shosa (Lieutenant Commander) Yoshiharu Soga expressed satisfaction with the new aircraft but express its doubts about the standing of this bomber on the long-run and that the Navy could not afford to remain complacent, urging to start development of a successor.

Informal discussions regarding this successor started in mid-1937. In September Specification 12-shi fwas drafted. It was for a "basic naval attack aircraft" with a tender for development that was abandoned. Technical specification were awarded to Mitsubishi, trusted to also design that successor. Main requirements of Specification 12-shi expressed a top speed of 215 knots (391 km/h) at 3,000 m, a range of 2,600 nautical miles (4,815 km) max, down to 2,000 nautical miles (3,700 km) with the same 800 kg payload. It needed to be powered however by the same two Kinsei engines rated for 1,000 hp, then already produced at Mitsubishi for the G3M. This was an absurd, spec. as increased in speed and range with the same engines only advocated for radical improvement of the airframe and fuselage. There was still a vague understanding of its operational use and of how a "maritime attack aircraft" was supposed to operate in the future. Mitsubishi's management and staff accepted the challenge and the final result was a superb product, based on unreasonable requirements.

Kiro Honjo led the development team as chief designer already of the G3M and of the experimental reconnaissance aircraft from spec. 8-Shi. Honjo was the most experienced aircraft designer in Japan for multi-engine aircraft. After reviewing specifications, Honjo concluded the best was to use a four-engine configuration if the same engines were retained. At the first development conference between Mitsubishi and Navy representatives, Honjo requested a revision of the specifications. He wanted to create a four-engine configuration, and supported this by his sketches a four-engine attack aircraft on the blackboard with chalk. Rear Admiral Misao Wada (head of the technical department) opposed this with vehemence however:

"...The Navy is capable of handling operational planning issues on its own! Mitsubishi should simply keep quiet and build a twin-engine attack aircraft in accordance with the Navy's specifications. Erase the drawing immediately!"

This ended Honjo's modern heavy bomber. His proposal nevertheless lingered in the minds of the IJNAF leadership. In 1937, unbeknownst to Mitsubishi, Kaigun Koku Hombu unofficially approached Nakajima to develop a next-generation "naval attack aircraft." in turn. From October 1937 there was a new chief at the head of Kaigun Koku Hombu's technical department, Rear Admiral Misao Wada. He instructed Nakajima's leadership to secretly acquire a Douglas DC-4E 44r-engine transport aircraft in secret and a license to manufacture it, officially for a new commercial company, Dai-Nippon Airline. Lengthy negotiations with Douglas, eventually succeeded and a DC-4E prototype was delivered. It became Nakajima's reference for the design, later evolving as the G5N1 "Shinzan" ("Liz"), but this remained unrealized with just six pre-production built, used as transports. Meanwhile, Mitsubishi continued working on the 12-shi spec. which included the installation of a 20mm defensive cannon in the tail, a last minute supplementary specification by December 3, 1937 after harsh lessons from the war in China that summer, as G3M bomber units were decimated.

The last minute tail cannon

The decision to install an aircraft cannon in the tail was confirmed in April 1938. The only remaining issue was finding the right ordnance, which seems to be the Type 94, produced in small quantities in Japan as a copy of the Swiss Oerlikon of 1930. It was however sunsuitable for the Navy as too bulky and with a low rate of fire, short range. By late 1937, a license for the new Oerlikon FF aircraft cannon was purchased for production at the Dai-Nihon Heiki KK in fixed and turret versions. But local producton never matched the awaited Swiss quality. Fine-tuning it took over a year and a half as the feeder constantly jammed, the barrel overheated. Instead, the MGFF with its rate of fire of 490 rounds per minute was accepted into service with the navy as the Type 99 Model 1 and gradually improved. There was also an attempt to convert the 12.7mm Ho-103 to a 20mm cartridge by late 1943, and the tail gun was replaced by a Ho-5. This gun turret was inside a glass cockpit behind the tailplane. The end result was a streamlined "cigar" hence the "hamaki" in Japanese. Kiro Honjo peferred "namekuji" (bullet).

A cigar or lighter ?

The next challenge was to find space for the 4,900 liters required to match the specified range. Kiro Honjo used integrated fuel tanks, with the wing having a two-spar structure enclosed within a box by skin load-bearing members for torsional rigidity. The interior was sealed and created a massive fuel tank. The wing skin was the only thing retaining all this fuel... Kiro Honjo rightly believed this would be extremely vulnerable to enemy fire unlike conventional fuel tanks using rubber lining and fire extinguishers but the Navy rejected it as it would reduce the capacity by approximately a third. The IJNAF showed again total stubbornness and shortsightedness, assuming all the risks. It was typical of the thinking of the time though for th top brass, achieving the objective at any cost, regardless of losses.

Bad lessons from China

The naval command were perfectly willing to sacrifice survivability for a very modest payload as long as the range was maintained and the crew was expandable. In China, bombers with high operational altitudes and good discipline with highly skilled crews and meeting initial weak resistance from Chinese aaviation tended to prove this point. This was even compounded by a more effective fighter escort. Naval command believed all this was sufficient to ensure the bomber's survival. But soon, the air war in China led to fatal consequences in the Pacific Campaign. Honjo's only victory was a concession when he obtained replacing the 1,000-horsepower Kinsei for Kasei radial under development at Mitsubishi. These were two-row, 14-cylinder, tested from September 1938 and producing an estimated takeoff power of 1,530 horsepower, for a massive performance boost.

Maiden Flight and fixes

On October 23, 1939, the "Hamaki" took off for a maiden test flight from Kagamigahara Airfield, north of Nagoya. At the commands were test pilots Katsuzo Shima and Harumi Aratani. Minor issues were noted about the handling, but overall, impressions were excellent. So much so that when completely light aerobatic were possible. With minor alteration, the new Mitsubishi bomber was accepted into service with the IJNAF on January 24, 1940 ad the G4M, succeeding the G3M. On February 27, a second prototype took off. It had a larger vertical tail surface and ailerons with trim tabs. Minor improvements were made to its steering and afterwards pilots expressed full satisfaction both woth the performances and handling. On March 15, this improved prototype was handed over to IJNAF for testing. The Navy Pilots achieved a top speed of over 240 knots (437 km/h) which exceeded specifications. Its unladen taxi range was 3,000 nautical miles (5,460 km), also exceeding requirements.

Towards the G6M1 "Escort Fighter"

The G3M1 was soon ready to enter production that spring of 1940. However events in China intervened and by May 17, 1940, the IJNAF launched "Operation 101", a four-month air offensive on Chongqing and other targets in Sichuan like Chengtu. Up to 130 G3M bombers were mobilized for these raids, but targets remained beyond their range, and the escorting A5M fighters were completely unable to provide support. This resulted over a sharp increase in losses and analysis revealed the bombers at the outer edges of the formations were the first downed, uncovered by crossing fire. The IJNAF command wanted then to develop some sort of "escort fighter" for based on the new G4M. Therefore, the decision was made to launch a limited-production of thse new G4M1 based twin-engine "escort fighters".

Mitsubishi however vigorously opposed the idea, as a bomber for them, whatever its merits, would not substitute a proper fighter. But again the Navy top brass enforced its decision and placed an order for 30 "escort fighters", officially called the "12-Shi Rikujo Kogeki Ki Kai" (Basic Naval Attack Aircraft Modified 12-Shi) or G6M1. The bomb bay was replaced by a large nacelle housing two 20mm cannons and partially shielding fuel tanks. The first two were completed in August 1940. Mitsubishi warned about the final results. As expected, the hastily converted bomber performed poorly. The increased drag caused by the massive gun pod and change of the center of gravity caused not only lower agility and imbalance, but also a steep increase in fuel consumption. The G6M1 was just unable to escort the bombers long-distance missions. The USAAF arrived a the same conclusion with their "gunships" YB-40 and XB-41. Still, the IJN staff continually pushed this idea until 1945.

By August 1940, as this ludicrous bomber conversion took place, another Mitsubishi model impressed everyone by its long range and overall agility. This was their new carrier-based fighter, the Mitsubishi Type 0 A6M. That phenomenal range was just what was needed to escort the new G4M bomber formations over China. The Zero had its first combat debut as soon as September 13, 1940, near Chongqing and reported to the IJNAF ended the G6M1's short career. The gun pod was removed, and the former "fightesr" were reconverted for training future naval aircrews. Meanwhile production resumed for the new G4M1 bomber. The G6M1 became the "Type 1 Large Base Trainer Model 11" or G6M1-K but its career was short and they were all converted into transports for 20 passengers, in service by October 1941 as the "Type 1 Basic Transport Model 11" of G6M1-L and to carry paratroopers.

Variants

In short:

G4M1 "Model 11": First production model, 1711 made
- G4M1 "Model 12": self-sealing fuel tanks
G2M2 "Model 22" Initial variant (274 built)
- G4M2 Model 22A: 20mm cannon dorsal (5 built)
- G4M2 Model 22B: 20mm Type 99 Model (2)
- G4M2A Model 24: 1,850 hp MK4T "Kasei"-25 (14)
- G4M2A Model 24a, same as Model 22a (15)
- G4M2A Model 24b same as Model 22b (171)
- G4M2A Model 24c: Nose 13mm "Type 2" HMG (1)
- G4M2B Model 25: G4M2A 24 with MK4V Kasei-27 1,795 hp (1)
- G4M2C Model 26: G4M2A24b test MK4T-V Ru "Kasei"-2bRu w/t turbochargers (1)
- G4M2D Model 27: G4M2 with MK4T-B "Kasei"-25b engines (1)
- G4M2E Model 24d mothership for the Yokosuka MXY7 Ohka (20+)
G4M3 Model 34": Shortened-range mod. with armored crew protection.

G4M1 Production and Modifications (1,170)

The first true production G4M1 bomber left the assembly line in December 1940. Its takeoff weight increased from prototype, from 6,48 to 7 tons. By then its top speed fell to 231 knots (420 km/h) and the range down to 2,315 nautical miles (4,210 km) when fully loaded. It still exceeded that of all contemporary foreign twin-engine bombers, even matching some four-engine heavy bombers. Trials at Yokosuka Kokutai went smoothly and it was accepted for service in April 2, 1941 as the "Basic Fleet Attack Aircraft Type 1 Model 11" better known as G4M1. From March 1942, the engines were upgraded to the Kasei Model 15 for high-altitude, from the 241st production aircraft onwards. From August 1942, from the 406th this became standard, alebti the external cowling remained the same. Soon reports of losses piled in however and the IJNAF tried to improve survivability.

Tests were made with wing fuel tanks setup with a CO2 filling system, but it proved ineffective. Even minimal damage to the wing skin resulted in leaks each time and the CO2 did not prevented ignition. From the 663rd model from March 1943 at last a 30mm-thick rubber sheet was installed on the lower outer wing surface and it managed to reduce the cruising speed by five knots and radius by 170 nm while the upper surfaces remained unprotected. Also, the tail gunner ammuntions were protected by two small 5mm-thick armor plate, but it still could not stop 7.7mm bullets, and were removed immediately. Othr changes comprised the propeller spinners, and slightly longer engine exhaust manifolds. In the spring of 1943, the tail gunner had a new truncated glazing cone for a better arc of fire. From August it was completely redesigned with a wider cannon port like on US gun mounts and a month later in September individual jet exhaust stacks replaced common manifolds, from the 954th aircraft onwards. Production of the Model 11 (G4M1) ended in January 1944, with 1,170 G4M1 delivered (plus the 30 G6M1s and two prototypes).

G4M2 Production and Modifications (1,142)

The G4M2 differed mostly by its more powerful Kasei Model 21 engines using a water-methanol injection system for 1,850 hp at takeoff. The engines also came with redesigned nacelles with individual exhaust pipes, and four-bladed propellers instead of 3-bladed. Called "Model 12" it was later changed to "Model 22". Armament was improved as well, and from November 1941 already it had been planned to replace the manual dorsal 7.7mm machine gun turret with a powered one with a 20mm cannon. But development was delayed and only arrived on production by early 1944. By then the takeoff weight rose to 12,500 kg from 9,500 kg, thus, the performances gain was modest.

The wings were also completely redesigned, and made thicker to accept a near-laminar airfoil profile and its internal fuel capacity increased to 6,490 liters but they only had limited rubber protection underside. The horizontal stabilizer was increased and reinforced. The tailwheel was simplified. No long retractable, it was fixed in the extended position. The glazed nose cone with a 7.7 mm machine gun had now a traverse of +/- 40° and elevation of +/- 45°. The nose had more extensive glazing, and aft of the wings, space was freed to install two additional 7.7mm machine guns whereas the side blisters of the G4M1 were replaced by flat and sliding glass panels. The tail gun mount had its protection ow made of small 10mm plates. The G4M2 prototype first flew on December 17, 1942 with three more prototypes made and testing showed a range of 3,031 nautical miles (5,500 km) and top speed of 236 knots (430 km/h).

The first pre-production and 5th prototype, was of Model 22, completed and tested from July 1943 and with the 6th retained for further testing due to issues found during the redesign. The most serious was a steady vibration from the new Kasei 21 engines, never fully resolved. From the 65th, the G4M2 was equipped with convex bomb bay doors, typically removed in the field. From the 105th a flat, transparent panel was installed under the nose cone to prevent distortion and enabled night raids. Here are some of the numerous variants of the G4M2:

Model 22 Ko:

Type 3 Kokutai Model 6 (H6) search radar, 7.7 mm MG on the sides swapped for 20 mm Type 99 Model 1. Side sliding panels over the gun ports redesigned, right one moved closer to aft.

Model 22 Otsu:

Turret-mounted 20 mm drum-fed Type 99 Model 1 cannon replaced with the more advanced Type 99 Model 2 with a longer barrel and belt fed.

G4M2A:

On May 22, 1944, the first prototype powered by Kasei Model 25 engines with a reduced gear ratio to address vibration issues took off. It was now faster at 243 knots (442 km/h) and started production as the Type 1 Model 24 (G4M2A). It also had new engine cowlings with separate carburetor intakes on top, modified bomb bay doors.

G4M2A Model 24 Ko:

This one was equipped with radar and 20 mm cannons in side mounts like the Model 22 Ko.

Model 24 Otsu:

It replaced its Type 99 Model 1 cannon in the dorsal turret with a long-barreled Type 99 Model 2.

Model 24 Hei:

7.7 mm machine gun in the nose replaced with a large-caliber 13.2 mm Type 2, in place of the radar antenna, moved up.

Model 24 Tei:

The G4M2E was modified to carry the MXY 7 Ohka kamikaze self-propelled jet bomb. The bomb bay doors were removed to accommodate this model and Mitsubishi records, showed it also had armored cockpits and fuselage fuel tanks. Fire protection for the wing tanks wa simproved with cavities filled with carbon tetrachloride, actively replacing oxygen with chlorine of hit and flaming out. This reaction also released toxic chlorine, and could potentially kill the crew of an internal tank was hit.

Model 25:

The G4M2B had the new Kasei Model 27 engines. Small numbers produced.

Model 26:

The G4M2C was a Kasei Model 25 Otsu with direct fuel injection

Model 27:

The G4M2D was powered by turbocharged Kasei Model 25 Ru engines from the Naval Arsenal. In all, a total of 429 G4M2s and 713 G4M2As, plus prototypes and variants were produced until June 1945.

Model 34 (G3M2)

The G4M3 was the last production model entering service with a reinforced armor and fuel tank protection, so it also had the shortest range. The wing structure was redesigned so to have now a single spar and small fillets in the wing roots. The first prototype was powered by Kasei Model 25 engines, and it flew on 01 January 1944. For this trials the outer rubber protection in the lower outer wings, causing drag, were removed, so it could reach still 259.7 knots (473 km/h). All 7.7 mm machine guns were now replaced with 20 mm cannons and the tail gun was now identiocal to the one from a captured Boeing B-17E. The fuselsage was shorter, as its center of gravity so the stabilizer angle was significantly increased.

Model 34 Ko

The Type 1 Model 34 Ko or G4M3A entered production in October 1944 but production was hampered by a strong earthquake combined with air raids over Nagoya so only 90, were deliveryed by 15 August 1945, from the plant in Misushima (Okayama Pref.). Subvariants included the "Model 34 Otsu" (siimilar upgrade as the Model 22 Otsu and Model 24 Otsu) and Model 34 Hei modified like the Model 24 Hei. The newt iteration was the Model 37 fitted with a turbocharger. Overall, total production all combined reached 2,435.

Design of the Mitsubishi G4M

Fuselage and general layout

This classic twin-engine with cantilever and all-metal monoplane was designed for a crew of 7-8. It had a near constant oval cross-section on the fuselage of 2.5 m x 2 m at the midsection and riveted as a monocoque structure. This superfifcially looked like a cigar, hence the nickname. The fuselage was made of 38 main frames with stringers and stressed skin in duralmunin wrapped aound. For ease of production as well as assembly, and repair, the entire fuselage was split at frame #24 and the Frames #12 and #14 were reinforced to accommodate wing spars. The bomber/navigator's glassed nose forward started at frame #1 to frame #5 and the pilot's cabin (to seats) located behind, at frames #6 and #11 below a glasshouse. There was an extra seat further aft to the right for an observer, and an enclosed section for the radio (right) and navigator's map table (left). The weapons bay was at frames #9 to #22. The radio operator also acted a dorsal gunner and could swap rapidly from its position to the dorsal dome. Behind the wing was located the flight engineer's station and second, side gunner position. The rear gunner's station was the most isolated, at the tail end glass cockpit, and he had to crawl above the bomb bay to go forward. The fuselage was appreciated however unlike the G3M as it had extra space amidship and aft. The crew enters through round door between frames #25 and #27, port side, often used as a marker to place the hinomaru. Since long range means also hours aloft, there was a lavatory located between frames #27 and #28, port side. Equipment was spartan to spare weight, but there was at least a Sperry autopilot and radio compass. The navigator could use a Type 90 optical bombsight located in the nose.

Wings

The wing is trapezoidal in overall like the Type 96 but less pronounced, and with rounded tips, smooth leading and trailing edges. The mid-wing position eliminates interference with the fuselage, without extensive fairings. This all-metal, two-spar structure featured a working skin with countersunk riveting. The wing profile was Mitsubishi MAC 118 along its span,that is 12.5% thick at the root, 10% at the tip. it was simplified to comprise five parts that could be all taken apart: A center section inside the fuselage with two inner consoles, and two outer consoles. The center section spars extended through the fuselage, attached to reinforced frames #12 and #14. The leading edges for the inner consoles are formed by the detachable oil tanks, and the trailing edge is equipped with slotted flaps. There was a metal frame and covering with each spanning 5.250 m, over an area of 4.108 m². They had electric drives.

Deflection angles are 36° on takeoff and 45° on landing. Attached to the trailing edge of the outer consoles are Fraize-type ailerons with a 15% aerodynamic compensation, with a metal frame and fabric covering for a span of 6.25 m, 2.05 m², and controllable trim tabs. Deflection angles were 18° upward, 27° downward. The engine nacelles were attached to the inner consoles and then to front and rear spars to be removed entirely.

Empennage

The empennage is cantilever as well, with a single vertical stabilizer whixh wingspan was ten meter, consisting of three parts, with a center section, and detachable outer wing panels. It had adjustable trim tabs on the control surfaces, metal structure and single-spar attached to frame #35 but fabric-covering for the control surfaces, with a spaon of 3.02 m², and deflection angles of 20° up, 25° down. The vertical stabilizer was made of two parts, a lower one attached to the fuselage, and upper one that was detachable with a reinforced rib showing the separation, metal structure all aroind but there was also a fabric-covered rudder. The trim tab was controlled on the rudder which spann was 1.82 m2, and had a deflection angle of +/- 30°. Commands by cables.

Engine and performances

The two original engines of the first version were Kasei-11 radial, two-row, 14-cylinder, and air-cooled engines. Their takeoff power was 1,530 hp/2,450 rpm and the supercharger was located in the second gear. Max power was 1,410 hp/2,350 rpm at 2,000 m or 1,340 hp/2,350 rpm at 4,000 m. There was a two-speed, single-stage supercharger for altitude flights. Each weighted 632 kg with a gear ratio of 0.684. They were provided with DC generators, to power the landing gear as well as the flaps, and other systems. These came with Sumitomo Hamilton CS-3B constant-speed propellers, which were all metal and three-bladed, 3.4 meters in diameter and with a pitch range from 20 to 40°. Each prop weighted 175 kg.

The engine cowlings were of the NACA-type with adjustable exhaust flaps. The exhaust manifolds were separate for each cylinder bank and their exhaust pipes routed to the upper surface of the engine nacelle. They had tunnel-type oil coolers, mounted inside the cowlings. The removable oil tanks were located on either sides of the engine nacelles, 150 liters each, anf forming the leading edge of the inner consoles.

The fuel system comprised a total of eight unsealed tanks for 4,780 liters between the wing spars, completed with their fuel lines and electric fuel pumps. Ther first two tanks had a capacity of 690 liters each, in the center section at least somewhat protected inside the fuselage and the inner ones were 630-liter, mounted on the wing consoles between the fuselage and the engine nacelle. There were two more tanks, with a capacity of 640 and 430 liters located between the engine nacelle and junction with the outer console, more exposed. Fuel fillers were located on the upper wing surface with also hatches on the underside for tank inspection.

The landing gear was fully retractable, electrically powered up and down as well as the tailwheel. There was a manual backup however. The main landing gear was a classic foorl type with hydraulic spring arms and attached to the front wing spar. They fully retracted forward, inside the the engine nacelles. In vase this failed, there was a simple gravity emergency release, using the oncoming airflow pressure. The main landing gear suspension was oleo-pneumatic. The wheels tires measured 1200 x 400 mmand were equipped with brakes. The tail wheel was a 500 x 180 mm tire, freely castering with a lock, retracting into the fuselage. In practice, it was rarely retracted abnd in fact the pilots removed the actuator and it was locked permanenlty in the extended position. On the G4M2, the retraction system was eliminated the the tail wheel became fixed to spare weight and complexity.

Armament

Bomb and torpedoes amounted to a symbolic 1,000 kg. They were housed in the weapons bay between frames #9 and #22 with doors closed only when the aircraft was not carrying any payload so in ferry operations or reconnaissance for better aerodynamics. When carrying torpedoes or large (800/500 kg bombs) these doors were open or partially removed. The chassis was large enough to support one 800 kgs or one 500-kg bombs, using identical attachement points. The torpedo section was as long as the bomb bay. To reduce airflow turbulence and improve bomb separation, there was a deflector installed at the rear of the bomb bay.

The reference torpedo was a unique Type 91 Kai 7 torpedo, weighing 1,055 kg. It carried a warhead of 323.6 kg (713 lb) HE or 235 kg (518 lb) for the mod 3. It was powered by a wet-heater type 8-cylinder radial engine rated for 150 kW (200 hp) for a top speed of 42 knots or 78 kph. It needed to be release no more than 2,000 m (2,200 yd) from the target. The crew prepared it by connecting it for constant electrical power and maintain compressed air as well as spinning the guidance mechanism gyroscopes, actuators, and engine during the flight. A simple release separated the umbilical. Bombs were suspended horizontally and apart one large bomb (see before) that could carry either four 250 kg, or twelve 60 kg bombs. In that case they were mounted on two bridge beams, in two rows of three each, attached rescectively to frames #10 and #14, and frames #15 and #19.

The defensive armament comprised four 7.69 mm machine guns plus two spare and the 20 mm cannon in the tail. One was were located in the navigator's cabin, one in the upper dorsal blister, two in the side blisters. The Naval Type 92 machine gun was essentially a copy of the Vickers in the same caliber abd worked with 97-round drum magazines or 47-round magazines in case the former were unavailable. The navigator's machine gun had a ball mount, asymmetrically in its rotating cone for better firing angles forward, rotated by an electric motor and handle. The deflection angles were 75° in elevation and 60° traverse. The other mounts were manually handed, and seven magazines of ammunition were carried. There was also a spare machine gun that could be mounted in a roof hatch from the navigator's cabin.

The dorsal blister (replaced by a turret in the G4M2) had a forward fairing and rear movable section to allow the entire mount to be deployed or relocated inside. The machine gun was attached to a transverse rai for a 70° elevation and 35° traverse either side. Seven drum magazines were also stored for them. The side blisters were similar in design with a 90° elevation and 60° traverse, also retratable inside and six drum magazines provided for each, located nearby. There was an additional spare machine gun that coild be moounted in a top hatch between blisters.

But the most potent defensive asset of the G4M1 was the tail gunner's Megumi Special Naval Type 99 Model 1 cannon. It had an oscillating mount allowing the gunner to maintain a constant position relative to the cannon when aiming vertically. It could simultaneously be manually rotated around its longitudinal axis. It had large 45-rounds stored behind the gunner and rolled to him with a conveyor belt. Elevation was limited to 35°, and traverse also 35° both sides.

Specs G4M1 Model 11
Crew:	7: pilot, co-pilot, navigator/bombardier/nose gunner, captain/top turret gunner, radio operator/waist gunner, engine mechanic/waist gunner, tail gunner)
Length:	19.97 m (65 ft 6 in)
Wingspan:	24.89 m (81 ft 8 in)
Height:	4.9 m (16 ft 1 in) in rigging position
Wing area:	78.125 m2 (840.93 sq ft)
Airfoil:	MAC118 mod (12.5%); tip:MAC118 mod (10%)
Empty weight:	6,741 kg (14,861 lb)
Gross weight:	9,500 kg (20,944 lb)
Max takeoff weight:	12,860 kg (28,351 lb)
Powerplant:	2× Mitsu. MK4A Kasei 11 14-cyl. A.C.R. 1,140 kW (1,530 hp) each TO
Propellers:	3-bladed Hamilton/Sumitomo constant speed variable-pitch
Maximum speed:	428 km/h (266 mph, 231 kn) at 4,200 m (13,800 ft)
Cruise speed:	315 km/h (196 mph, 170 kn) at 3,000 m (9,800 ft)
Stall speed:	120 km/h (75 mph, 65 kn)
Range:	2,852 km (1,772 mi, 1,540 nmi)
Ferry range:	5,040 km (3,130 mi, 2,720 nmi)
Rate of climb:	9.17 m/s (1,805 ft/min)
Guns:	1× 20 mm Type 99 tail, 4× 7.7 mm Type 92 LMG fus.
Payload:	858 kg/800 kg bomb or 4× 250 kg bombs or Yokosuka MXY-7 Ohka.

Operational career of the Mitsubishi G4M

The G4M was similar in performance and missions to other contemporary twin-engine bombers such as the German Heinkel He 111 and the American North American B-25 Mitchell. These were all commonly used in anti-ship roles. The G4M Model 11 was prominent in attacks on Allied shipping from 1941 to early 1944, but after that it became increasingly easy prey for Allied fighters.

By early 1941, the IJNAF had six Kokutai dedicated for the G4M, each was named after its permanent peacetime base. The "Kisarazu" and "Kanoya" Kokutai were the oldest, formed in August 1937, with G3M bombers, and proud of their history. From 1938, the "Kisarazu" Kokutai retrained and by January 1940, moved to the east coast of Tokyo Bay. With its long runways, it proved was ideal for operating large aircraft, and hosted most bomber by the late 1930s and early 1940s. All were trained here. The "Kanoya" Kokutai was a combat unit located on the southern coast of Kyushu. The southernmost base was "Takao" Kokutai based in Takao, Formosa (Taiwan), part of the Empire of Japan since 1895. The "Chitose" Kokutai was facing Alaska and the Kuriles from Hokkaido. Two new naval bomber units were formed in late 1940, "Mihoro" Kokutai in northern Hokkaido and "Genzan" Kokutai in Genzan (Wonsan) in Kore, Chosun Province.

To consolidate the strategic command, the 11th Air Fleet (Koku Kantai) was established on January 15, 1941, having all bombers units under its control. On April 10, two additional Kokutai were formed, the 1st and 3rd tokusetsu ("special units") on specific operations and on temporary as forward bases. The first was formed at Kanoya, the other at Takao. Each Kokutai (abbrev. Ku) comprised four hikokitai or "hikotai" with their own aircraft, crews, support making them fully autonomous and mobile. Kokutai were essentially "air corps" but they were closer to regiments or small divisions. but surely were duvided into squadrons or "buntai", or chutai (used interchangeably albeit they covered various aspects). The hikotai was really the closest to a full "squadron" with all its components. Its commander was the buntaicho, seconded by a chutaicho managing the unit ground assets. Eacg comprised usually nine bombers and three in reserve; to match the smallest formation of three in rotation.

The later was shotai (flight), but the number of shotai in a hikotai varied over time between three and six, each led by a hikotaicho, led by a single hikocho, captain 3rd rank in the IJN acting as second-in-command for the kokutai. The CO or kokutaicho was a IJN captain 2nd rank. In May 1941, Takao kokutai was considered the best prepared and was the first to receive the G4M. By July and August 1940, G4M1s were also delivered to the Kisarazu kokutai for training and start producing new crews for more deliveries to come.

Combat Debut in China

"Operation 101" in 1940 already showed the limits of the use of bombers on distant targets in China. By July 1941, the IJNAF launched another such air offensive, "Operation 102" committing the entire 11th Air Fleet (180 bombers based at Hankou) for a raid on the Sichuan Province, Chongqing and Chengdu. But as relations with the United States were rapidly deteriorating it was decided to complete the conquest was one last massive strike to free these bombers for upcoming operations linked to the "southern strategy", now agreed by the majority in the interarm staff and sanctioned by Generalissmo H. Tojo. The Takao kokutai flew into Hankou on July 25, bringing there 30 G4M1s, and two days later, they took off for Operation 102, raiding Chengdu. But before getting there, they were intercepted by a massive Chinese resistance. In 1939-40 indeed, the Chinese Nationalist AF hd been reinforced by the latest US aiacraft, not counting volunteer pilots and mercenaries, and they took a heavy tall on the incming G4M1s as they already did on the G3Ms.

Fortunately by that stage, the IJN could now escort these formations with a few A6M fighters, just arroved that summer of 1940. WWhen present, they demonstrated complete air superiority over any Chinese aircraft, so much so the latter deliberately choose to avoid combat. The Japanese wanted to lure out these fighers to finish them off and tried various "bait formations" but the Chinese planners knew the game and carefully choose when to launch their forces. Long story short, the new G4Ms could maintain a cruising speed of 170 knots, about the same as the A6M, and those from the Takao kokutai provided navigation for the Zeros in escort, but the bulk of the G3Ms from other units would arrive at the target later. Operation O-go ("102") took several days, until August 11th and committed twenty A6Ms from the 12th Kokutai from Yichang, closest to the Chinese positions. Nine G4Ms from Takao Kokutai under the command of Lieutenant Yogoro Seto left Hankow at 01:35 on August 11 and they reached Yichang by flying low not to be signalled, made them known, so that the Zeros took off in complete darkness, lined up next to the G4Ms. One formation of two bombers and four fighters separated for a diversionary attack on Hanzhong and Guangzhong. The remainder headed for Chengdu, and arrived before dawn at 05:05. The Chinese had been warned and sent their Tupolev SB bombers from Wen Chang away just before the Japanese arrived, but they sent in interception all I-153 fighters from 4th sqn. Shuang Lui.

Six I-153s arrived first, four shot dow by Zeors. Over Wen Chang airfield, the Japanese bombed and destroyers two Tupolev remaining there broken down. In this first pass, five I-153s were claimed destroyed in the air. Complete success, with no losses. Some G4M1s in this mission acted as leaders for the Zeros without any bombs and so could not do much against the Chinese airfield. This was still a Japanese feat of night, long distance flying and coordinated discipline. Western analysts doubted the distance ran by the G4M1s from Hankou, as the distance was almost 100 km further than London to Berlin. No bomber of that range existed in Asia it was believed.

Combat Debut in the Pacific

By July 1941, Japanese forces attacked southern French Indochina and the US froze all Japanese assets, soon joined by Dutch and British Commonwealth, and they also cut off all oil supplies. The Japanese started preparing the seizure of the Dutch East Indies by force to get these resouces and Operation 102 was terminated on August 31, freeing bomber units in Hankou to return to their bases in Japan by September 1-2. The IJNAF, except a few recce aircraft, withdrew entirely from China until the fall of 1941. The Army and IJAAF were now in charge. Naval pilots started intensive training for these new operations.

Back to Kanoya Air Base, corresponding Kokutai was converted in turn to the G4M. The 3rd Kokutai became a fighter unit with the new A6M and the last G3Ms were retired. The 3rd Kokutai had previously four G3M buntai between Takao and Kanoya and get six G4Ms squadrons. In October the 1st Air Fleet (Koku Kantai) was prepared for the attack on Pearl Harbor. On land, operations were left to the 11th Air Fleet. Bases in Taiwan were prepared and the 21st and 23rd koku sentai (air flotillas) were tasked of attacking the Philippines and in general be called in support for all the southern campaign areas. From bases in southern French Indochina, the 22nd koku sentai was tasked of supporting the Malayan conquest and the advance toward Singapore. In the Philippine it was at first planned to only use carrier fighters but given the experience in China, crossing the 500 nautical miles from bases in southern Taiwan, to Manila and back was retired to the carriers. The 3rd and Tainan Kokutai were prepared to use Zeros for very long-range raids over the ocean. An aspect that was worked hard on, was the timing of the attack on Pearl Harbor by taking in account time zone differences with the US to avoid giving the impression of a surprise attack without official notification. But as we known there was an error and that what's happened. G4M and A6Ms crews underwent intensive coordination training in the fall of 1941 for these ultra-long-range raids, unprecedented in the history of combat aviation.

On October 31, 1941, the 24th Koku Sentai, inc. the Chitose Kokutai, was detached from the 11th Air Fleet, assigned to the 4th Fleet and tasked with capturing Guam and Wake, central Pacific. The G3Ms of the Chitose Kokutai already completed redeployment to the Marshall Islands by late October. By mid-November, the still unique G4M unit was reassigned to the Chitose Kokutai, based from Rua, Kwajalein, under LCt. Nobuo Ando.

Raids on the Philippines

On October 25, the British famously sent Force Z, HMS Prince of Wales and Repulse from the Home Fleet to Singapore, and on November 4, the US air command transferred six B-17Cs and 29 B-17Ds to Clark Field, Luzon, Philippines wiht more expected in December. On November 18, the 1st Kokutai was stationed in Tainan and soon joined by the Kanoya Kokutai at Taichu with the HQ 22nd Koku Sentai based in Saigon, Indochina to direct the Genzan and Mihoro Kokutai. On November 30, the Kanoya Kokutai transferred half its complement to Saigon when it was learned Force Z had passed Colombo, and were now closing on Singapore. The transport Keiyo Maru was loaded at Takao with aviation fuel and type 91 mod.2 torpedoes for the Kanoya Kokutai, departing for Indochina. On December 2, Prince of Wales and Repulse arrived in Singapore. The Kanoya Kokutai started its transfer with a selection of the bewt Crews, selected individually, as the most experienced in torpedo attacks by Captain Second Rank Naoshiro Fujiyoshi. He was at the head of 36 G4M1 bombers (27 + 9 reserve) from the 1st, 2nd, and 3rd Chutai.

Stormy weather over the South China Sea caused flight issues but by December 5, the transfer was completed and on the 7th, it flew to Thu Dau Moi, 20 km NE of Saigon. The 11th Air Fleet had at that stage 216 twon-engine bombers, excluding reserve aircraft, now half were G4Ms and the other half G3Ms. G4Ms of the Takao and Kanoya kokutai, 54 each. 1st Genzan and Mihoro kokutai each had 36 G3Ms. The Chitose Kokutai had 27 G3Ms, one G4M1 at Rua, Marshalls only used for long-range reconnaissance. On Sunday, December 7, the attack on Pearl Harbor was launched, but the main attack on the Asian side wuuld take place on Monday, December 8.

G4M1 of Takao Kokutai 753rd unit, Rabaul October 1942. The improvized camouflage is mostly field-made over the base primer sliver grey, sprayed vermicels in black, dark grey and red brown (pinterest).

The 11th Air Fleet scrambled all its squadrons to take off from bases in Taiwan at 1:30 AM. After a long night flight they reached their target shortly after dawn, c6:30 AM. Twenty-seven G4M1s from Takao and 27 from Kanoya escorted by 54 A6Ms from the 3rd and Tainan Kokutai attacked Nichols Field, on the outskirts of Manila. Another 27 G4Ms from Takao kokutai, 27 G3Ms from the 1st kokutai, escorted by 36 A6Ms from Tainan kokutai bombed Clark Field, 100 km closer. At 10:30 PM on December 7 in a dense fog on the coast of Formosa, started to spread to the Takao area and at 1:00 AM on December 8, Vice Admiral Nishiso Tsukahara, 11th Air Fleet commander scrambled a top staff reunion as intercepted US radio messages indicated US forces were redeployed north of the Manila along a defensive line to Iba and Clark. Coupled with fog, this delayed the takeoff ofthe second wave and forced Tsukahara to abandon the attack on Manila.

Plans to Clark Field remained unchanged but not Nichols Field. 54 G4M bombers were redirected on Iba airfield. Later the fog over Tainan started to clear. At 7:50 a.m., order to take off was received with the G3M being the first to take off at 8:15 a.m. Zeros took off an hour later. Over Takao, the fog cleared at 9:30 a.m so the 54 G4Ms from Takao and Kanoya Kokutai took off, followed by Zeros and passed Mount Niitaka, highest peak in Taiwan and excellent landmark for orientation, seen in the morning light. Meanwhile the attack on Pearl Harbor was already in full swing and strikes on Davao underway while IJN Ryujo struck Mindanao. Northern Luzon already had been attacked and the bombers were on their way back. Surprise had been lost for the initially first wave, becoming a 2nd wave due to the fog. US fighter pilots from scattered P-40s & P-36s in the Philippines were all in high alert.

Half of the Takao units led by Commander Yoshiso Suda, observer in the lead Betty piloted by Jiro Adachi, and 26 bombers led by their hikocho Irisa, headed for Iba and the other half led by hikotaicho, Taro Nonaka plus G3Ms from the 1st Kokutai, headed for Clark Field at an altitude of 7,000 meters. US fighters had been in the air since early morning, unaware of the weather problems in Taiwan, saw after no sign of the Japanese around midday, they started to land for fuel, just when Japanese bombers appeared. At 12:36, they attacked, dropping some six hundred and thirty six 60 kgs bombs (42 tons) on Clark Field in a textbook precision. This destroyed twelve B-17s, damaged five more (later written off) as well as twenty P-40Bs. Iba Airfield was hit by four and eighty-six 60-kg and twenty six 250-kg bombst minutes after Clark, claiming Seven P-40Es on the ground, and the radar. Meanwhile Zeros shot down six fighters. 50% of the US bomber force in the Philippines and 35% of the fighter force. The US pilots still managed to shot down seven Zeros, one Betty from Kanoya Kokutai, which survived only to crash-land near its base after the long return flight.

Destruction of Force Z

Meanwhile G3Ms of the 22nd Koku Sentai took off in a thunderstorm for their first raid on Singapore while the G4Ms from Kanoya kokutai assigned to search for Force E remaining on the airfield on high alert, awaiting orders to attack. A Type 98 C5M2 "Babs" confirmed Force Z was in Singapore at 9:50 A% and conformed by another reconnaissance aircraft. Decision was made to destroy them in port that night. However at 3:40 p.m., I-65 claimed they had already departed and were now in the South China Sea at 1:45 PM. It turned out that the "Babs" crew mistaken two large tankers for them. The IJN was soon aware the two ships were now underway to intercept the Japanese invasion force. Vice Admiral Sir Tom Phillips indeed had sailed at 5:35 PM on December 8 with four destroyer escorts and were underway to Singora, southern Thailand. Vice Admiral Jisaburo Ozawa ordered the naval bomber force to take off and search for them in the Saigon area. Thios was foiled by a sharp deterioration in the weather and darkness. Rear Admiral Matsunaga decided to postpone the all-out search and wait for the following day. At 6:44, G4M1s from three hikotai of Kanoya took off from Dau Moi airfield loaded with the recent Type 91 Kai 2 torpedoes capable of shallow diving, with more powerful warheads. The weather improved significantly and meanwhile Admiral Phillips was forced to abort the Singora mission when spotted by a floatplane from a Japanese cruiser and turned back to Singapore. At 10:00, Genzan Kokutai bombers spotted them and signalled their position and heading. At Saigon, the 22nd Koku Sentai HQ immediately scrambled the coordinates to all units already in the air.

The faster G4Ms from Kanoya Kokutao missed the message and at 10:28, they were 600 nm from base, Singapore 80 nm to their right, Sumatra ahead, turning around and heading back to base. Rear Admiral Matsunaga eventually sent British coordinates in plain text at 11:30 PM and units northwest of Anambas Island turned toward the assembly point. LtCdr. Shichiso Miyauchi in the lead aircraft, piloted by Senior Lieutenant Miyoshi Nabeta led the nine aircraft of the 1st Chutai (Moritaka Higashi) and 3rd (Lieutenant Haruki Iki), reaching the assembly point, but could not see a thing due to dense clouds. In a break Miyauchi spotted a glimpse of a floatplane flying below, which was a Supermarine Walrus from HMS Repulse. The formation then went down through the clouds and at 12:15, broke through down to 800 meters,and after 2 minutes sighted Force Z. Repulse was 2,500 meters behind Prince of Wales, an the destroyers were deployed on POW's starboard. From 11:15 to 11:57, a total of 32 "Nell" (G3Ms) bombers and torpedo bombers attacked in successive waves. Repulse suffered a single 250-kg bomb hit but sailed on and thanks to brilliant handling Captain Tennant dodged no less than 15 torpedoes in a comb.

Those of Genzan KKt. attacked the Prince of Wales, managed two torpedo hits on her port side, one hit, disabled the rudder, jammed the port propeller shafts and damaged its power system so she could no longer use its 133mm DP guns albet she managed to shoot down one attacker. Down to 10 knots and unable to maneuver, she was now a practice target for the G4Ms. Nine from the first chutai (Miyauchi) approached down to 300 meters and at 8 km, emerged from the clouds, spotted by the British in a patch of bright blue sky, trigerrng a barrage of AA fire, but at 12:18, Miyauchi flapped his wings twice to signal the attack. Miyauchi launched first against the Prince of Wales on her exposed starboard. Repulse at 28 knots was caught in a starboard turn toward the attackers, unaware that Prince of Wales lost control. It was assumed the flagship would also turn and they themselve started a left turn in anticipation only to realize she was still straight ahead, loosing momentum and having to return for a seocnd pass. The second and third flights abandoned the attack and reformed.

Miyauchi's first shotai continued on its original course and dropped their torpedoes when reaching 1,000 meters from the target, even 500 meters, so at 12:20, two torpedoes struck Prince of Wales at the bow and forward of the bridge. One was dropped too late and missed. The 2nd and 3rd Shotai made another run on Repulse, always turning teir bow to them, heeling heavily but the aircraft won the race, caught her in a pincer, launching two starboard, three port. Repulse was hit once to port close to aft superstructure, but still was making 25 knots. One G4M, unable to get into a good position to attack Repulse, switched to Prince of Wales and hit her starboard. The 2nd chutai (Higashi) approached Repulse with 6 bombers from the 1st and 2bd shotai from the port side, with two re-routed on Prince of Wales, from the starboard, making one more hit aft. Prince of Wales finally lost power, started listing starboard.

Repulse, managed to evade six torpedoes, only to succumb to the third chutai (Haruki Iki) from her rear, first assessing the situation and assisting his chutai to focus all attention on the battlecruiser. After a full circle, they attacked from the port side, six other from the starboard side and torpedoes were dropped from 800 meters at 30 meters. However two were shot down. Repulse suffered a hot on her port side near the engine room, one aft of the aft main battery turret, one at the aft end and a hit to starboard near boiler room "E". Two minutes later, Captain Tennant ordered to abandon her. HMS Repulse capsized and sank, stern first at 12:33 PM. Prince of Wales lingered for 30 min. longer, before two G3Ms dropped 500-kg bombs, one hitting amidships at 12:43, and then minutes later, she capsized and sank with Vice Admiral Phillips and Captain Leach.

The IJN pilots knew they just made history. Until then capital ships had been sank at anchor, Taranto and Pearl Harbor. There, they proved that they could sink them underway, at full speed, alive and kicking. This also doomed all chance sof stopping an invasion forces. Singapore was left to its defenders. Losses were comparatively very lmight, two "Betty" and one "Nell" from Genzan KKt and one heavily damaged and, belly-landing southwest of Soc Trang after running out of fuel, three other "Betty", one "Nell" from Mihoro damaged but later repaired.

Supporting the Offensive

The desire to seize the rich natural resources of the Dutch East Indies, with the capture of Java as the ultimate goal, led to the development of Japanese offensive plans along two main axes. In the east, the Japanese hoped to break through the Philippine archipelago, reaching the oil fields of the coast of Borneo, Sulawesi, and other islands, and heading toward eastern Java. In the west, their route lay south through Malaya to Singapore, with the goal of establishing a strong foothold on the west coast of Borneo and southern Sumatra, and then approaching the western tip of Java.

Philippines Campaign:

On December 10, bad weather forced three of the "Kanoya" Kokutai hikotai to remain grounded in Taichu, but the "Takao" Kokutai appeared over Luzon in full force. Half of the 27 G4Ms kokutai were tasked with completing the failed raid on Nichols Field, south of Manila, on December 8, while the other half, an equal number of Bettys, were to attack Del Carmen Field, south of Clark Field. Clouds over the target prevented the second group from attacking, forcing them to turn their attention to shipping in Manila Bay. The Zeros escorting the bombers engaged P-40Es over Manila, destroying the remnants of the US 24th Fighter Group.

By December 12, the weather around Formosa and the surrounding waters had improved, but parts of Luzon Island were still under cloud cover. When 27 Bettys from the Kanoya Kokutai were sent to bomb Clark Field, they found the entire airfield obscured by clouds and were forced to unload over Iba Airfield. A Takao Kokutai with 52 Bettys was diverted to attack Batangas Airfield due to fog over Nichols Field. The next day, a total of 104 Bettys bombed various targets on Luzon Island, 26 Kanoya Kokutai aircraft and another 26 from the 1st Kokutai bombed Nichols Field, permanently disabling it, and 52 bombers from Takao Kokutai attacked Olongapo, Iba, and Del Carmen airfields. In less than a week of fighting, the Japanese had gained overwhelming air superiority over the Philippines, and they were already scrambling for viable targets.

Moreover, the "Betty" units, reliably covered by the ubiquitous "Zeros," suffered virtually no losses. G4M bombers—the "long arms of the Imperial Navy"—operated successfully in the south, both in the Philippines and beyond. Thus, on December 18, a "Kanoya" Kokutai unit, with 25 "Type 1 Rikkos," flew from Taiwan to Peleliu Island, in the Palau archipelago, to support the landing in Davao scheduled for December 20, 1941. On December 21, 21 "Kanoya" Kokutai "Type 1 Rikkos," taking off from their new base on Peleliu, attacked Del Monte Airfield on Mindanao Island.

On December 22, 1941, the main Japanese invasion force in the Philippines landed at Lingayen Gulf, on the island of Luzon. In the following days, the Japanese Army achieved significant successes in providing tactical air support to its own Army Air Corps. This allowed the Philippines campaign to be left solely in the hands of Army Aviation, while Naval Aviation shifted its focus to other missions. For example, bomber units from Formosa hunted shipping near Manila from December 25 to 28. G4M crews then turned their attention to the Bataan Peninsula and the island fortress of Corregidor. However, it should be noted that the Japanese Army abandoned Allied air power in the Philippines early on. For example, on December 25, the 90th Sentai of light bombers, consisting of 27 Ki-30s, taking off unescorted on a ground attack, was suddenly attacked by 11 P-40s from the remnants of the US 24th Fighter Group, which shot up 18 aircraft in mid-air and damaged six more.

Corregidor was a serious threat; it was the strongest and most well-defended target the Japanese had encountered in the early stages of the Pacific War. The island fortress's extremely powerful air defenses forced experienced G4M crews to retrofit their aircraft with Kasei 15 engines. These engines significantly increased the Betty's operational ceiling, operating over enemy territory riddled with anti-aircraft guns. During one raid on January 3, 1942, one Betty, a Takao Kokutai, made an emergency landing on Vigan Island after an anti-aircraft shell struck its fuel tank. The following day, another Betty was shot down by one of the few remaining P-40s over Bataan. On December 28, 1941, an advance group of 10 Kanoya Kokutai "Betty" fighters flew to the newly captured airfield in Davao, on the southern coast of Mindanao. The rest of the aircraft arrived there by January 5. The island of Jolo, halfway between Mindanao and North Borneo, was captured on Christmas Day, and between January 2 and 8, Takao Kokutai and its 23 "Betty" fighters flew to the local airfield on the island. Although American and Filipino forces continued to hold Bataan and Corregidor, the Japanese were rapidly advancing toward their intended objective—the Dutch East Indies.

Malaya and Singapore:

(To be continued in Part II...)

Last combat Days

On August 19, veteran naval bomber pilot Taii (First Lieutenant) Den Sudo of the Yokosuka Kokutai took off from Kisarazu Air Base in command of two Bettys. His mission was to deliver the Japanese delegation to the surrender negotiations. One of the aircraft Sudo piloted was a veteran G6M1-L transport, and the other was an old friend, the Type 1 Rikko Model 11 (G4M1). On orders from General Douglas MacArthur's headquarters, both aircraft were painted entirely white. Dark green crosses were painted on the upper and lower wings, fuselage, and vertical tail surfaces in place of the hinomaru insignia. The aircraft were to deliver a 16-person delegation, led by Chōshō (Colonel General) Torashiro Kawabe, to the tiny island of Le Chima near Okinawa. There, General Kawabe and his delegation were to board a USAF C-54 transport plane and fly to Manila to receive detailed instructions from the Allied command regarding surrender.

The Japanese aircraft crews waited for the delegates overnight at Le Chima airfield, and after the delegates returned from Manila the following day, Sudo piloted his plane carrying General Kawabe and ten of his companions on the return journey. An unexpected fuel leak during the flight forced it to make an emergency landing near the mouth of the Tenryu River. Fortunately, no one was injured. The second aircraft remained at Le Chima for another day for minor repairs, but it soon returned, safely reaching Kisarazu Air Base with the remaining members of the delegation. Thus ended the last official mission of the G4M "Type 1 Rikko" "marine attack aircraft."

One of the most famous livery was for the model 11s that carried the delegation to Ie Shima island on Aug.19, 1945 for the preliminaty surrender talks. This was a converted transport model, with a livery following allied guidelines. Note on the photo below, the original upper camouflage still shows through.

According to the final instructions from the American command, the Imperial Navy was to disband its Special Attack units first, with particular attention given to the "Jinrai" (Divine Thunder) unit of the 721st Kokutai. Given the unit's specific mission—carrying Okha bombs—and the strong patriotic sentiment within the unit, the IJNAF command sought to implement the victors' will as quickly as possible, fearing a repeat of the uprising that had occurred at Atsugi Air Base in the first days after the surrender. On August 23, 1945, the G4M bombers took off for the last time from Komatsu Air Base in Ishikawa Prefecture, the final resting place of the 721 Ku. Once again, the base was filled with the sound of Kasei engines as the Rikkos taxied for takeoff. They scattered to airfields across Japan, carrying their surviving sons home to their families.

Gallery

G6M1, Naval Air Tech Arsenal Late 1940

G4M1 761 Kokutai Kanoya 1943

G4M1 #383

G4M1 #337

G4M2 in night bombing colors,Kokutai 762 Hikotai Kogeki 1944

Books

Bunrin Do - Famous Airplanes Of The World 59 - Mitsubishi G4M Betty (translated as much as we could)
Japanese Aircraft of the Pacific War page 378-387
Japanese Aircraft of World War II : 1937-1945 ,Thomas Newdick, Edition 2 2017, page 25-28
(Wydawnictwo Militaria No.67) Mitsubishi G4M “Betty” | PDF
(Aircraft Profile 210) - Mitsubishi G4M Betty PDF | PDF
MARU MECHANICS Yokosuka P1Y & Mitsubishi G4M Betty | PDF
Bunrindo - Famous Airplanes of The World 59 - Mitsubishi G4M 'Hamaki ’ Army Type 1 Attack Bomber