The Mitsubishi K3M was designed by Britishaeronautical engineer and aircraft designed Herbert Smith, from Sopwith working in Japan for Mitsubishi. The prototype, designated Mitsubishi 4MS1, made its maiden flight in 1930. The aircraft was strut-braced high-wing cabin monoplane, with fixed wide-track landing gear, and was powered by a single 300 hp (220 kW) water-cooled radial piston engine. Pilot and gunner were located in separate open cockpits, with an instructor and two pupils in the enclosed cabin in the fuselage. Later passenger variants seated five passengers in the cabin.
Total production of all versions was around 625 aircraft, with production mostly undertaken by Kyushu Hikoki K.K. and Aichi Kokuki. Production continued until 1941, and examples pressed into service as liaison aircraft in the postwar period were found in a variety of national markings.
The first version of the K3M offered to the Imperial Japanese Navy Air Service was prone to stability problems, and more importantly, problems with the water-cooled 340 hp (250 kW) Mitsubishi-built Hispano-Suiza eight-cylinder liquid-cooled engine driving a two-blade wooden propeller.
The improved K3M2 used a Hitachi Amakaze 11 nine-cylinder air-cooled radial engine, rated at 340 hp (250 kW) for take-off and 300 hp (220 kW) at sea level, driving a two-blade wooden propeller. The first K3M2 production version entered service in 1932 as the Navy Type 90 Crew Trainer. It was superseded in production with the K3M3, using a Nakajima Kotobuki 460 hp (340 kW) air-cooled engine.
The Navy Type 90 Crew Trainer was primarily a land-based aircraft; however, a few were converted with floats for use as seaplanes.
The Imperial Japanese Army Air Force had an interest in the aircraft as part of its modernization program, and as a potential supplement to the Nakajima Ki-6. Two examples were acquired and tested, and the airframe was given the designation of Ki-7. One prototype used a 475 hp (350 kW) Mitsubishi Type 92 nine-cylinder air-cooled radial engine and the other a 450 hp (340 kW) Nakajima Kotobuki nine-cylinder air-cooled radial engine. However, the IJAAF opted not to go into production with either version.
The civil version offered to commercial operators was with a 420 hp (310 kW) Nakajima-built Bristol Jupiter VI nine-cylinder air-cooled radial engine. The Mitsubishi K3M was used for both civil and military roles with some remaining in operation until well after World War II.