The Mitsubishi B1M was a Japanese Navy torpedo bomber that was designed by Herbert Smith, formerly of Sopwith, and that served well into the 1930s. Like many Japanese companies Mitsubishi decided to enter the aircraft industry by learning directly from the west. Smith, and a design team of former Sopwith men, moved to Japan, where they produced a series of aircraft, including the 1MF fighter and the 2MR reconnaissance aircraft. They also designed a torpedo bomber, with the designation 2MT (2 seat Mitsubishi Torpedo aircraft).
The 2MT was a standard biplane of wooden construction, with two open cockpits and powered by a Napier Lion inline engine. It made its maiden flight in January 1923, and entered Navy service as the B1M1 Type 13 Carrier Attack Bomber.
The B1M remained in service much longer than had been planned. The Japanese Navy began work on a replacement aircraft in 1927, but the Mitsubishi B2M was delayed until 1932 and was disappointing when it arrived, the Yokosuka B3Y was a failure and the 7-Shi specifications of 1933 failed to produce a suitable aircraft. The B1M was finally replaced in the mid 1930s by the Yokosuka B4Y, another biplane produced in response to a set of 9-Shi specifications and only ever seen as a stop-gap measure while work continued on the Nakajima B5N 'Kate', which entered service in 1938.
These delays meant that the B1M was still in front line service during the Shanghai Incident of 1932. In January 1932 the 1st Air Wing, on the carriers Kaga and Hosho launched a series of attacks on Shanghai. One B1M was lost on 22 February when a formation of three attack aircraft and their fighter escorts were attacked by an American volunteer, Robert Short, flying a Boeing Model 218 (the export version of the P-12E). Short was also shot down.
2MT1/ B1M1/ Type 13-1 Carrier-Attacker
The prototypes and early production versions, powered by a 500hp Napier Lion engine, were given the Navy designation B1M1 Type 13-1 Carrier Attacker.
Prototype and initial production version
500hp Napier Lion engine
2MT2 and 2MT3/ B1M1/ Type 13-1 Carrier-Attacker
The same naval designation was given to the 2MT2 and 2MT3, two versions with minor changes. A total of 196 B1M1s were built
The 2MT4 Otori was a twin-float reconnaissance version of the aircraft. Three were built for tests, but the type didn't enter production.
The 2MT5/ B1M2 was given a 500hp Hispano-Suiza V12 engine. A total of 116 were built
3MT2/ B1M3/ Type 13-2-2 Carrier Attacker
The 3MT3/ B1M3 was a three-seat version of the aircraft, powered by a 600hp Hispano-Suiza V12 engine. One of the fixed forward firing machine guns was removed. Mitsubishi built 88 3MT2s in 1929-30, and these were followed by 40 built by the Hiro Arsenal, for a total of 128 3MT2s and 440 B1Ms of all versions.
The designation T-1.2 was given to a number of civil conversions of surplus B1Ms. They were given enclosed passenger cabins with two or three seats, located behind the pilot's position.
Specifications (2MT2/ B1M1)
Engine: Napier Lion
Wing span: 48ft 5.5in
Length: 32ft 0.75in
Height: 11ft 5.75in
Empty Weight: 3,179lb
Maximum take-off weight: 5,946lb
Max Speed: 130mph
Service Ceiling: 14,76ft
Armament: Two fixed forward firing 0.303in machine guns and two pivoting 0.303in machine guns
Bomb-load: One 18in torpedo or two 529lb bombs