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The Junkers K 51/Ki 20 was a modification of the G 38 passenger aircraft of the German Junkers Flugzeugwerk AG with the aim of constructing a bomber for the Imperial Japanese Army Air Force.
The Japanese army acquired the licensing rights as early as 1928 – even before the completion and maiden flight of the G 38 – and in 1931 commissioned Junkers to design the conversion of the G 38 passenger aircraft into a large bomber, which was to be built in Japan as the Type 92. The Japanese aircraft manufacturer Mitsubishi Nainenki Kabushiki Kaisha was commissioned to implement the project. The Dessau designers designed a prototype type with the designation Junkers K 51, which showed only very minor changes to the G 38 passenger aircraft already produced. The blueprints were accepted by the Japanese in 1932. In the same year, production of the bomber under the name Ki-20 began at Mitsubishi under the strictest secrecy. With the support of the Junkers engineers and technicians, an all-round defence system with machine guns and a device for transporting 5000 kg of bomb load, which had already been developed in Dessau, was integrated there. In addition to command posts in the fuselage and in the nacelles of the external engines, which are pulled behind the wing edges, two downward-extending weapon posts were also installed in the wings. As with the G 38, the maiden flight was carried out by Junkers chief pilot Wilhelm Zimmermann. The type was already obsolete when it was introduced, which is why production ended after six Ki-20s were built. For this reason, the Ki-20 was never used for live bombing operations until its retirement in 1941.

A project of the Mitsubishi company from 1930 to Development of a heavy bomber based on the Junkers G-38/K-51. It was a Mid-decker with cabin for the crew, four 12-cylinder in-line engines, Four-blade propellers, box stabilizer and fixed, partially faired chassis.



In 1930, Mitsubishi acquired the license to build a replica of the Junkers G38 passenger aircraft and its bomber version K-51. The Army Air Force agreed to the construction of two prototypes. This resulted in a machine that is largely adapted to the K-51. The crew consisted of 10 men. Eight men were in the hull , one shooter sat in a double fighting position on the hind wing in extension of the outer engines. These consisted of one downwards each directed, rotating machine gun stand and a backward and upward-firing machine gun in the a combat post at the rear wing end. In addition to the pilot and co-pilot, there were two men to support the movements of the control surfaces. All four were sitting in a pulpit on top of the fuselage at the front end of the wings. Right and left in a machine gun was installed in each pulpit. On the bow was a combat stand with machine gun This was replaced in 1934 by the Type 94 20 mm autocannon. The bomber was housed under the bow combat post. He aimed over bearing marks in a wooden rack under the bow. Another Combat post with machine gun was installed on top of the fuselage at the height of the wing center. A Mechanic completed the crew in the fuselage. All combat positions were open. Due to the high weight, the landing gear consisted of two tires arranged one behind the other. Up to 5000 kg of bombs could be carried openly under the fuselage and wings .
Two prototypes were built in 1932 from self-made and imported parts. In the process, some detailed improvements have been made among other things, on the chassis and the cooling system During the tests, the machine proved to be as underpowered, but the usefulness as a strategic bomber as a outstanding. Therefore, the official introduction took place in 1932. However, the costs per machine were immense, so that only a small series was built. . Until 1935, Mitsubishi therefore produced only 4 other aircraft of the type. The machines were classified as top secret. Accordingly, they did not initially receive a Ki number and were not classified as official operational aircraft. It was not until 1936 that the prototype designation Ki-20 even though the plane was already outdated by then. The background to this was everything to confuse with the term foreign observers. A modern Successor model was already in development at that time.
After 1933 the engines were replaced by four in-line Jumo 204 engines, each with 750 hp Standard power replaced. While this improved performance, the However, underpowering remained a problem. Therefore, tests with Kawasaki Ha-9 engines, but they were not successful.
Until 1938, this type formed the core of the strategic bomber fleet of the Army Air Force. The main task would be in the event of a war with the Soviet Union, the bombing of strategic targets in Siberia . For this purpose, the machines were held back until they were retired. From 1938 they were replaced by the Ki-21. With an operating weight of almost 26 t and 5 t bomb load, the Ki-20 was the heaviest aircraft and the one with the heaviest Bomb load that has ever been put into service by the Army Air Force.