Mitsubishi B2M (1927)

Imperial Japanese Navy ship-based torpedo bomber (1927-1937), 206 manufactured.

The Mitsubishi B2M was a Japanese carrier-based torpedo bomber of the 1920s and 1930s. It was built by Mitsubishi to a design by Blackburn Aircraft of Britain and was operated by the Imperial Japanese Navy. A bridge between biplanes and monoplanes, never replacd after the failure of the B4M in 1934. It was used for bombing at high and low-level against China during the early phases of the Second Sino-Japanese War (1937) and was replaced by the Nakajima B5N "Nate" monoplane.

The pre-production Blackburn design was called the T.7B and it was essentially an enlarged development of the Ripon. Later this became the B2M, built under licence by Mitsubishi and with scores of modifications, starting with the engine. It was just not only the "Japanese Ripon". The prototype T.7B first flew on 28 December 1929 in Great Britain at Blackburn's test field at Brough, Yorkshire, in front of the Japanese delegation. It was approved, and it was shipped, dismounted, to Japan, with a set of Hispano engines, in February 1930 to be copied under licence.

Development

Prototype T7B
Since 1902, there was an alliance between Britain and Japan, based on a mutual defiance towards the Russian Empire, that ther former saw a expansionist threat ti its own Far East Empire. Even after the Russo-Japanese war, crippling for the Russian Pacific fleet (and causing the loss of the Baltic fleet to boot) this alliance remained valid. Japan built all its battleships and battlecruisers in britain until the Satsuma and Kawachi class, Japanese officers and trained in Britain and the Royal Navy was a model for doctrine, tactics and everything in between. When the race was on to built the first dedicated aircraft carrier, Japan managed to complete IJN Hosho faster than britain's Hermes. Even post-WW1 Britain was essential for the development of Japanese aviation, with missions sent to Japan from manufacturers to help building the young IJN aviation branch. The army, by contrast, started to look more at France and Germany on that matter.

In the meantime, Sopwith worked with Mitsubishi to create the successful 1MF1 (1923) designed by British engineer, Herbert Smith (and so was the Mitsubishi 2MR reconnaissance aircraft). Since the IJN had written specifications for a torpedo-bomber as well, Mitsubishi came out with the 1MT, by the same Herbert Smith, somewhat inspired by the Sopwith Cuckoo, but much larger and made as a triplane, as the Japanese wanted it equipped with their large and heavy 533 mm naval torpedo.

However this turned out as a failure, proingd difficult to fly and unable to operate from an aircraft carrier when carrying a torpedo. The type was soon withdrawn and scrapped after 20 were delivered. Despite of this experience, Mitsubishi was still eager to work with Smith, and when a new specification to replace it was emitted, Smith worked on an alternative design based on the stretched-out 1MF mixing aspects from the 2MR. The Mitsubishi Type 13 became the B1M and every bit a success for the company with a long service and large production (443).

However back in Britain, Sopwith already closed its doors in September 1920 (hence the departure of Smith for Japan) and the only two companies that could provided naval aircraft in Britain were Fairey and Blackburn. But the latter turned to be, after a delegation was sent to Britain, to be praised for its well-engineered, well-thought "marine" aircraft, making Blackburn the true "naval air specialist" in Britain at the time. Mitsubishi was well aware that Blackburn was its preferred choice, but it made a ver unusual move. When an official competition to replace its own B1M came out, it took part in the competition in Japan while launching its own competition in Britain...

Mitsubishi's Internal Competition

Indeed, despite Mitsubishi's B1M good work for Japanese carrier-based torpedo aviation, the IJN staff realized that this successful and robust design no longer met performance expectations and announced a competition for a new torpedo plane. Requirements were that the new design be capable of a flight speed of at least 200 km/h, on a wingspan of less than 15 meters, length of less than 10 meters, maximum height of less than 3.8 meters, tailored to the carrier elevators of the time. BMW, Hispano-Suiza, or Lorraine engines were to be used going from 450 to 650 hp. With that engine, it had to climb to 3,000 meters in less than 15 minutes.

Manufacturers included Mitsubishi, Nakajima, Aichi, and Kawanishi. Mitsubishi approached the new project in an unconventional manner as it launched own competition, submitting its own proposal to three British design offices, Herbert Smith from ex-Sopwith now, after being an independent consultant, working now at Mitsubishi, but also Blackburn and Handley Page. Smith presented Mitsubishi with its own design for the 3MT3 powered by a 650 hp Armstrong-Siddley Leopard engine, not specified by the Navy. Handley Page offered the 3MT5, Blackburn ccame out with its 3MT4, a modified version of the Blackburn Ripon. The latter two chose the Hispano-Suiza engine. However Mitsubishi initially rejected Smith's proposal, and later in 1924 he returned to UK and retired from the aviation industry. This was the end of Sopwith influence in Mitsubishi.

Mitsubishi's Ripon

Inded, of the three designs, Mitsubishi selected the 3MT4 from Blackburn, for further development. Mitsubishi commissioned a flying prototype based on their plans, which was quickly made, completed and first flying on December 29, 1929. It was then was disassembled and shipped to Japan where it arrived in February 1930. Blackburn engineer George E. Petty accompanied it to facilitate Mitsubishi's team "reverse engineering" and creation of its internal propototypes. He participated indeed in the reassembly of the aircraft and construction of the second prototype 3MR4. The 3MR4 (three prototypes would be made) was powered by a modernized version of the Hispano-Suiza Type Hi engine, the Hispano-Suiza 12Lbr, rated for 650 hp. This first 3MR4 N°1 was completed on October 31, 1930. It was not destined for test flights, as it crashed after a pilot error before its official tests. Partly rebuilt, it was used for static tests. By February 2, 1931, the third prototype was completed and has flight tests at home and then in front of a commission of the the Navy for approval.

Mitsubishi built three prototype 3MR4 in Japan (added to the reassembled T.7B) with a new engine, before the production version was adopted as the Navy Type 89-1 (Model 1) carrier attack plane, also known as B2M1 according to the terminology code of the time. The B2M2 later appeared, but overall production span was shorter as already in 1933 the company worked for the next specifications, based on the competition 7-shi. It was tried by Mitsubishi with its 3MT10, but failed as only two prototypes were made (navy model B4M) loosing to the Yokosuka B4Y. Nakajima would win the next round with the B5N (Type K) in 1937. Mitsubishi will return to fighters and beat Nakajima at its own game with the A5M and later legendary A6M, and never produced a torpedo bomber again, albeit manufacturing the carrier and land-based bomber B5M for the Navy in 1937 in small numbers and the main IJN twin-engine torpedo bomber, the G3M "Nell" and the next G4M.

Design

General Layout

The T.7B was a three-seat biplane of steel tube construction, wrapped in canvas, with high aspect ratio wings B-9 aerofoil fitted with Handley Page slats. It was already powered by a 466 kW (625 hp) Hispano-Suiza 12Lbr engine and considered and enlarged Ripon. It featured a strutted main landing gear with a tail skid. The B2M was a also much heavier model than the B1M, at 2,670 kg (5,886 lb) versus 1,442 kg (3,179 lb) and more powerful also, with still an inline engine, the Hispano-Suiza 12Lbr V-12 water-cooled piston engine rated for 485 kW (650 hp) versus a Napier Lion W-12 rated for 370 kW (500 hp), without much difference in speed however, at 213 km/h (132 mph, 115 kn) versus 210 km/h (130 mph, 110 kn). Only the engine cover was all metal up to the observer's position, then tubes and canvas, as the wing frames, tail and ailerons.

The Crew was a flexible two to three. In all cases was the pilot forward with his own seat and cockpit opening, then a large "bath-tub" for an observer and/or gunner that could acts as radio operator depending on the missions and roles chosen. The B2M was a longer aircraft at 10.27 m (33 ft 8 in), with a Wingspan of 15.22 m (49 ft 11 in) but fortunately both wings were folding. Its height was 3.71 m, testing already the limits of hangar roofs. This model was had a loaded weight of 3,600 kg, and also became the heaviest carrier-borne model in use.

Power and Performances

B2M2 Taking off from a carrier

Its inline was the Engine Hispano-Suiza 12Lbr V-12 manufactured under licence by Mitsubishi and mater to a wooden a two-bladed propeller. It was rated for 650 hp, for a top speed of c213 km/h (132 mph) at sea level. Its service ceiling was 4,500 m and range 960 km (1,779 km range sometimes quoted). The speed improvement over the B1M was negligible, but this was not a required improvement. The Navy mostly wanted it capable of handling well with a heavy load, be a stable platform to drop a torpedo at low altitude and have a great range.

Problems however were identified on the B2M1 like horizontal stability, hampering landing maneuvers on carriers. Pilots complained of its erratic behaviour and poor vibility. Handley Page fins were advised to be fitted, and a modernized tail assembly for a fourth prototype which eliminated these stability issues. In March 1932, the prototype N°4 finally approved and entered production under the designation B2M1 but carrier pilots however still preferred to fly the proven Mitsubishi B1M.

The main issue after introduction became the modernized Hispano-Suiza engine, which was prone to frequent breakdowns, an issue especially when flying over open water. Mitsubishi thus would develop the B2M2 featuring further modifications to the tail assembly and modified wingtips. But it did not meet the expectations of its users, especially since its performance was not significantly better than that of the Mitsubishi B1M model it was supposed to replace...

Armament

The B2M was not unarmed indeed. In addition to a forward-firing fixed standard 7.7 mm (.303 in) Type 92 machine gun it had a rear flexible 7.7 mm (.303 in) machine gun in the cockpit, with ready cheesebox ammunition racks for quick change. It carried either 800 kg (1,764 lb) of bombs, likely four 200 kgs models under belly racks using the torpedo's fixing straps, or a standard IJN airborne 45 cm (17.7") Type 91 torpedo, mod 1 (1933). The latter weighted 1,728 lbs. (784 kg) for an overall Length of 208 in (5.275 m), a negative Buoyancy of 227 lbs. (103 kg) and carrying a 331 lbs. (150 kg) Type 97 payload. It was powered by a Kerosene-air wet-heater for 140 HP and unique setting of 2,200 yards (2,000 m) at 41-43 knots. In remained in service ten years after service introduction, notably used by the G3M Nell to sink HMS Prince of Wales and HMS Repulse on 10 December 1941.

Length:	10.27 m (33 ft 8 in)
Wingspan:	15.22 m (49 ft 11 in)
Height:	3.71 m (12 ft 2 in)
Wing area:	55 m2 (590 sq ft)
Empty weight:	2,670 kg (5,886 lb)
Gross weight:	3,600 kg (7,937 lb)
Powerplant:	Hispano-Suiza 12Lbr V-12 water-cooled piston engine, 485 kW (650 hp)
Maximum speed:	Maximum speed: 213 km/h (132 mph, 115 kn)
Endurance:	1,779 km (1,105 mi, 961 nmi)
Service Ceiling:	4,500 m (14,800 ft)
Armament:	forward firing, flexible 7.7 mm LMG rear, 800 kg (1,764 lb) torpedo/bombs
Crew	3: Pilot, Radio/Navigator/Bombardier, Gunner/Observer

Variants

Production total for both versions: around 204–206 aircraft.

B2M1 (Type 89-1): initial production version approx. 1930-33.
B2M2 (Type 89-2): revised version, shorter wingspan, modified tail, other improvements. Produced 1933-35.

Operational career

The B2M1 entered service around March 1932 aboard carriers such as Akagi, Kaga, and Hōshō. The B2M (both versions) was used for bombing (high- and low-level) against China during the early phases of the Second Sino-Japanese War in 1937. The initial version was partly financed and delivered (donated) via the Hokoku-Go program. Despite being a new design in its era, it was regarded as not a very successful aircraft in service, suffering from engine reliability issues/limited performance compared to newer types. The B2M2 tried to correct some of these issues, but performances did not improved much. Pilots still hated it and preferred the B1M. This explains the relatively limited production and quick replacement

B2M2 over China, 1937, Ray Wagner coll.

After its frontline service, it was relegated to training or shore-based duties. The Navy released it after 1937 to the civilian market as Mitsubishi Type 89 as a general purpose aircraft, stripped of all military gear and hook, wings fixed. It remained in civilian service in various roles until around 1939 or beyond.

The B2M marks an interesting phase in Japanese naval aviation, one in which Japan still relied on foreign (British) designs/licences, while building up its own manufacturing and design capability. It served aboard Japan’s early carriers and formed part of the fleet-air arm’s inter-war development, helping bridge the gap to more advanced types later used in World War II. From a design-history standpoint, it’s an example of biplane torpedo bomber technology transitioning out as monoplanes and faster types emerged.

Gallery

B2M1 Omura Kokutai 1934

B2M2 IJN Kaga 1936

B2M2, IJN Kaga China August 1937

Type 89-1 Carrier Attack Aircraft B2M1 Hosho Air Group

B2M1 in 1932

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