Mitsubishi Ki-21

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 , endurance of at least five hours, and a bombload of . The design parameters were very ambitious for the mid-1930s; 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 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. ==Operational history==

The Ki-21-Ia was used in combat in the war with China by the 60th Sentai from autumn 1938, carrying out long-range unescorted bombing missions in conjunction with the BR.20 equipped 12th and 98th Sentais. The Ki-21 proved to be more successful than the BR.20, having a longer range and being more robust and reliable. Two more Sentais, the 58th and 61st deployed to Manchuria in the summer of 1939 for operations against China, with aircraft from the 61st also being heavily used against Russian and Mongolian Forces during the Nomonhan Incident in June–July 1939. This was followed in production by the Ki-21-Ic with provision for a auxiliary fuel tank, fitted in the rear weapons bay and one more machine gun, bringing the total to six. Four bombs were carried externally. To offset the increase in weight, the main wheels of the Ki-21-IC were increased in size. at Yontan Airfield, 25 May 1945 Towards the end of the war, remaining Ki-21s were used by Giretsu Special Forces in strikes against American forces in Okinawa and the Ryūkyū Islands. One of the noted operations was an attack on the Allied-held Yontan airfield and Kadena airfield on the night of 24 May 1945. Twelve Ki-21-IIb's of the Daisan Dokuritsu Hikōtai were dispatched for a strike, each with 14 commandos. Five managed to crash-land on the Yontan airfield. Only one plane landed successfully. The surviving raiders, armed with submachine guns and explosives, then wrought havoc on the supplies and nearby aircraft, destroyed of fuel and nine aircraft, damaging twenty-six more. A number of Ki-21-Ia were modified to serve as military transports for use by Greater Japan Airways, which was under contract by the Japanese Army in China for transportation. Designated "MC-21", these aircraft had all armament and military equipment removed. Used primarily as cargo transports, each could also seat nine paratroopers. Aircraft built from the start as transports were given the separate designation of Mitsubishi Ki-57, with equivalent civil aircraft being designated MC-20. Code Names The Ki-21 had more than one Allied codename. Initially called "Jane", the name was quickly changed to "Sally" when General Douglas MacArthur objected that the name was the same as that of his wife. When the Ki-21-IIb entered service, the absence of the long dorsal "greenhouse" led Allied observers to mistake it for a completely new type, which was designated "Gwen". However, when it was realized that "Gwen" was a new version of the Ki-21, it was renamed "Sally 3", with "Sally 1" referring to the earlier Ha-5 powered models, and "Sally 2" referring to the Ha-101 powered Ki-21-IIa. ==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 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 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 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 ;B.Th.4:() Royal Thai Air Force designation for the Ki-21-I. ==Operators==

; • Imperial Japanese Army Air Service • Imperial Japanese Airways (Dai Nippon Kōkū) ; • 6 operated by the Manchukuo Imperial Air Force ; • Royal Thai Air Force operated nine Ki-21-I Nagoya Postwar ; • French Air Force (''Armée de l'Air'') operated at least one Ki-21 as a transport in Indochina. ; • Indonesian Air Force ==Specifications (Ki-21-IIb)==