MItsubishi Ki-21, part one

 

Back in 1936, the Imperial Japanese Army issued a requirement for a new heavy bomber to replace the Mitsubishi Ki-20 and the Mitsubishi Ki-1 bombers which were clearly outdated for the time. The requirement called for a crew of at least four men, a top speed of 400 km/h (250 mph) with an endurance of at least five hours and a bomb payload of 750 kg (1.650 lb). Those technical requirements were very ambitious for the time and very few twin-engine bombers could achieve such performance numbers.

Mitsubishi and Nakajima were asked to submit their proposals, with Kawasaki's one being rejected. Mitsubishi's design was an all-metal mid-wing cantilever monoplane with a retractable landing gear, ventral bomb bay and two Mitsubishi Ha-6 radial engines. 

The first prototype flew for the first time on 18th December 1936, with the second prototype, which differed from the first one in having a long greenhouse canopy instead of a ventral turret, flying in January 1937. In the ensuing competition against Nakajima Ki-19 (Nakajima's proposal), they found to be similar in performance numbers, being the Ki-21 better in speed and endurance and the Ki-19 being better as a bombing platform and its Nakajima Ha-5 engines being more reliable. In order to make a decision, two more prototypes were ordered from both Mitsubishi and Nakajima, with Mitsubishi ordered to switch its own 825 hp Ha-6 engines for the Ha-5 ones, used by Nakajima and vice versa. That third prototype of the Ki-21 featured a glazed nose similar to the Ki-19 and revised tail surfaces. Therefore, the Ki-21 proved to be superior and was ordered into production as the "Army Type 97 Heavy Bomber Model 1A" in November 1937.

Production aircraft began to enter service in August 1938, supplementing and replacing the Fiat Br.20 Italian bombers that had been purchased as an interim measure. 

The Ki-21-Ia was used in combat for the first time during the Second Sino-Japanese War by the 60th Sentai (an unit equivalent to an aviation Group or Wing in other air forces) from autumn 1938, performing long-range unescorted missions together with the Br.20 of the 12th and 98th Sentais. The Ki-21 proved to be more reliable, robust and better suited for the long Asian distances than the Br.20. Two additional Sentais, the 58th and the 61st were deployed to Manchuria in the summer of 1939 to operate against China, with aircraft of the 61st being heavily employed against Soviet and Mongolian forces during the Nomohan incident (AKA Battles of Khalkin Gol) in June-July 1939, when the Japanese forces launched their main attack.

Sources:

1. https://en.wikipedia.org/wiki/Mitsubishi_Ki-21

2. https://www.valka.cz/Micubisi-Ki-21-Sally-t31499 (translated)