The Mitsubishi Ki-21, formal designation "Type 97 Heavy Bomber" (九七式重爆撃機, Kyūnana-shiki jūbakugekiki) was a Japanese heavy bomber during World War II. It began operations during the Second Sino-Japanese War participating in the Nomonhan Incident, and in the first stages of the Pacific War, including the Malayan, Burmese, Dutch East Indies and New Guinea Campaigns. It was also used to attack targets as far-flung as western China, India and northern Australia. The Allies designated it under the reporting names "Sally" /"Gwen". In 1936, the Imperial Japanese Army Air Service issued a requirement for a new heavy bomber to replace both the Ki-20 (Army Type 92 Heavy Bomber) and the Ki-1 (Army Type 93 Heavy Bomber). The design called for a crew of at least four, top speed of 400 km/h (250 mph), endurance of at least five hours, and a bombload of 750 kg (1,650 lb). The design parameters were very ambitious for the mid-1930's; few twin-engine bombers anywhere in the world could exceed such performance at that time. Both Mitsubishi and Nakajima were asked to build two prototypes each, a further proposal from Kawasaki was rejected. The Mitsubishi design was an all-metal mid-wing cantilever monoplane with retractable landing gear, ventral bomb bay and two radial engines. The first prototype flew on 18 December 1936, with the second prototype, which differed in replacing the dorsal turret of the first prototype with a long greenhouse canopy, following later in the month. In the resulting competition, Mitsubishi's Ki-21 and Nakajima's Ki-19 were found to be similar, with the Ki-21 having better performance, while the Nakajima design was a better bombing platform and had more reliable engines. In order to make a final decision, two further prototypes were ordered from both Mitsubishi and Nakajima, with Mitsubishi instructed to change its own 615 kW (825 hp) Mitsubishi Ha-6 radial engines for the Nakajima Ha-5 engines used by the Nakajima design and vice versa, while the Ki-21 gained a revised glazed nose similar to that of the Ki-19 and revised tail surfaces. Thus modified, the Ki-21 proved superior and was ordered into production in November 1937 as the "Army Type 97 Heavy Bomber Model 1A". Production aircraft began to enter service in August 1938, supplementing and then replacing the Fiat BR.20 bombers which had been purchased as an interim measure. Several improved versions followed before the production of the type ended in September 1944. A total of 2,064 aircraft were built, 1,713 by Mitsubishi and 351 by Nakajima. Variants Ki-21 Prototype models with various engines and armament combinations for evaluation. Final version with Nakajima Ha-5 engine. 8 built Ki-21-Ia (Army Type 97 Heavy Bomber, Model IA) First production model, with 634 kW (850 hp) Nakajima Ha-5-kai engines. Most were built by Mitsubishi, 143 manufactured by Nakajima Ki-21-Ib (Army Type 97 Heavy Bomber, Model IB) Improved version with additional 7.7 mm (0.303 in) machine guns, larger bomb compartment and flaps, redesigned tail. 120 built by Mitsubishi, 351 (including Ki-21 Ib and Ic) by Nakajima Ki-21-Ic (Army Type 97 Heavy Bomber, Model IC) Improved type with one additional 7.7 mm (0.303 in) machine gun, increased fuel capacity, 160 built by Mitsubishi Ki-21-II Evaluation model with more powerful engines, 4 built Ki-21-IIa (Army Type 97 Heavy Bomber, Model IIA) Production model based on Ki-21-II, 590 built Ki-21-IIb (Army Type 97 Heavy Bomber, Model IIB) Final production version based on Ki-21-IIa with modified canopy, clear upper cabin replaced by rotating turret, 688 built. MC-20-I (Army Type 100 Transport, Model I) Unarmed civilian transport version, converted from Ki-21-Ia; approximately 100 aircraft were converted.
