Back in 1938 when the Ki-21 entered service with the Imperial Japanese Army Air Force (IJAAF) its capabilities attracted the attention of the Imperial Japanese Airways (often referred to with their Japanese acronym of DNKKK - Greater Japan Airways Co.). Therefore, a civilian version of the Ki-21 was developed. This had many aspects in common with its bomber counterpart, as it retained the Nakajima Ha-5 KAI radial engines rated at 950 hp each and some other details. It differed mainly in having the same wings moved down from a mid-wing to a low-wing configuration and the incorporation of a new fuselage which could accomodate up to 11 passengers. The civilian variant of this new version was called MC-20, while the military version was called Ki-57.
The first prototype of the Ki-57 flew for the first time in July 1940 and, in spite of a fatal crash suffered by the fourth prototype, it was ordered into production both for the IJAAF, the DNKKK and the Imperial Japanese Navy Air Force (IJNAF) which employed a small number of Ki-57-II, and was known as the L4M1. A total of 101 Ki-57 and MC-20s were manufactured between June 1940 and April 1942 at Mitsubishi's factory in Nagoya.
The Ki-57-II was an improved variant powered by two Mitsubishi Ha-102 rated at 1.080 hp each, which also powered the Ki-21-II bomber. This variant also featured the revised engine nacelles that the bomber had. They were also 25 mph (40.23 km/h) faster than the Ki-21-I, had a higher service ceiling and could carry heavier loads. Production of the Ki-57-II began in May 1942 and it ran until January 1945 with a total of 306 machines completed (though, according to other sources, the total produced number was higher; 406).
The Ki-57 was, numerically speaking, the most important transport plane of the IJAAF, its main role the Ki-57 was employed in was communications and logistics, but it was also used in paratroop transport and saw service in every front where the Imperial Japanese Army was involved. Its biggest success came on 14th February 1942 when Ki-57s (together with same Kawasaki Ki-56s) of the 1st Raiding Air Regiment (Teisin Sentai) transported paratroopers to attack the oil refineries at Palembang, on Sumatra to prevent their destruction by the Allies before they fell into Japanese hands.
As the war progressed, and the Allied increased their air superiority, the Ki-57 proved to be very vulnerable to enemy fighters. Dozens were lost when the Allies swept West, including many that encountered heavily armed American patrol bombers, which were not afraid to engage the defenseless transports. When the war ended, many surviving Ki-57s and L4M1s were repainted in white with green crosses painted over the Hinomaru (the Japanese roundel) and were used to ferry Japanese delegates to the surrender ceremonies.
In 1942 every version of the Ki-57 received the Allied nickname of "Topsy".
Sources:
1. https://en.wikipedia.org/wiki/Mitsubishi_Ki-57
2. http://www.historyofwar.org/articles/weapons_mitsubishi_ki-57.html
3. https://pacificeagles.net/mitsubishi-ki-57-type-100-topsy/
4. https://www.valka.cz/Micubisi-Ki-57-Topsy-t33121 (translated)
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