Mitsubishi Ki-57 (Topsy)
When in 1938 the Mitsubishi Ki-21 heavy bomber began to enter service with the Imperial Japanese Army, its capability attracted the attention of Japan Air Lines. In consequence a civil version was developed and this, generally similar to the Ki-21-I and retaining its powerplant of two 950 hp (708 kW) 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 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 Mitsubishi 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 L4Ml. After the last of the Ki-51s had been delivered production was switched to an improved Ki-57-II, which introduced more powerful 1,080 hp (805 kW) 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, and 406 were built before production ended in January 1945. Both versions were covered by the Allied codename 'Topsy'.