In 1938, when the Ki-21heavy bomber began to enter service with the Imperial Japanese Army, its capability attracted the attention of the Imperial Japanese Airways. In consequence a civil version was developed and this, generally similar to the Ki-21-I and retaining its powerplant of two 708 kW (950 hp) Nakajima Ha-5 KAI radial engines, differed primarily by having the same wings transferred from a mid to low-wing configuration and the incorporation of a new fuselage to provide accommodation for up to 11 passengers. This transport version appealed also the navy, and following the flight of a prototype in August 1940 and subsequent testing, the type was ordered into production for both civil and military use.
This initial production Ki-57-I had the civil and military designations of MC-20-I and Army Type 100 Transport Model 1, respectively. A total of 100 production Ki-57-Is had been built by early 1942, and small numbers of them were transferred for use by the Japanese navy in a transport role, then becoming redesignated L4M1. After the last of the Ki-57s had been delivered production was switched to an improved Ki-57-II, which introduced more powerful 805 kW (1,080 hp) Mitsubishi Ha-l02 14-cylinder radial engines installed in redesigned nacelles and, at the same time, incorporated a number of detail refinements and minor equipment changes. Civil and military designations of this version were the MC-20-II and Army Type 100 Transport Model 2, respectively. Only 406 were built before production ended in January 1945. Both versions were covered by the Allied reporting name "Topsy".
Ki-57-I Army Type 100 Transport Model 1: Powered by two 708 kW (950 hp) Nakajima Ha-5 KAI radial engines and a redesigned fuselage to accommodate 11 passengers. About 100 aircraft of this type were built including the civil version.
MC-20-I: Same as above but built for civil use with Imperial Japanese Airways (Dai Nippon Koku KK).
Ki-57-II Army Type 100 Transport Model 2:Powered by two 805 kW (1,080 hp) Mitsubishi Ha-l02 14-cylinder radial engines installed in redesigned nacelles. Minor equipment and detail refinements were also incorporated. 306 aircraft of this type were produced before the end of production in January 1945.
MC-20-II: Same as above but built for civil use with Imperial Japanese Airways (Dai Nippon Koku KK).
L4M1: A small number of Ki-57-Is were transferred for use by the Japanese navy as transports and were redesignated L4M1.