In the late-1920s, as Junkers developed the Junkers G.38, Mitsubishi representatives in Germany expressed an interest in a military version of this civilian transport. At the time, the G.38 was the largest landplane in the world. Junkers completed a design study for a military bomber/transport, based on the G.38, designated the K.51. This design was not accepted by the Reichsluftfahrtministerium.
The K.51 design study was of interest to Japan. A licensing and manufacturing agreement was reached and in 1932, the first two Ki-20s were completed by Mitsubishi, utilizing Junkers-made parts. A prototype was successfully flown in Japan by a German test pilot in that year.
Four additional Ki-20s were built between 1933 and 1935. All of these subsequent models used Mitsubishi-built parts. Ongoing development focused on engine upgrades to all examples to address the persistent issue of the aircraft being underpowered. Several engine upgrades were completed during the lifetime of these aircraft. The initial Junkers L88 engines were replaced by the more powerful Jumo 204 engines, also built under license by Mitsubishi. Additionally, Kawasaki Ha-9 engines were utilized for testing purposes.
During World War II, the Japanese originally intended to utilize the Ki-20s to attack the forts at the entrance to Manila Bay in the Philippines, as well as for deep penetration missions into Siberia. For these purposes, they were armed with six gun positions and structurally enabled to carry a bomb load. These aircraft were the largest operated by the Imperial Japanese Army Air Service and their existence within it was kept secret. As a result, they were issued their out-of-sequence Kitai number '20' only when they were finally revealed in 1940.
A single example survived to the end of hostilities as a museum piece in the Tokorozawa Aviation Memorial Hall. All examples of this aircraft were either destroyed during the war or broken up for scrap during the latter portion of the 1940s.