| Ki-21 | |
|---|---|
 Mitsubishi Ki-21 | |
| General information | |
| Type | Heavy bomber |
| Manufacturer | Mitsubishi Heavy Industries |
| Primary users | Imperial Japanese Army Air Service |
| Number built | 2,064 units (excluding Ki-57) |
| History | |
| Introduction date | 1938 |
| First flight | 18 December 1936 |
| Retired | 1945 |
| Variant | Mitsubishi Ki-57 |
TheMitsubishi Ki-21, formal designation "Type 97 Heavy Bomber" (九七式重爆撃機,Kyūnana-shiki jūbakugekiki) was a Japaneseheavy bomber duringWorld War II. It began operations during theSecond Sino-Japanese War participating in theNomonhan Incident, and in the first stages of thePacific War, including theMalayan,Burmese,Dutch East Indies andNew Guinea Campaigns. It was also used to attack targets as far-flung aswestern China,India andnorthern Australia. TheAllies designated it under thereporting names "Sally" /"Gwen".
In 1936, theImperial Japanese Army Air Service issued a requirement for a new heavy bomber to replace both theKi-20 (Army Type 92 Heavy Bomber) and theKi-1 (Army Type 93 Heavy Bomber).[1] 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.[2]
Both Mitsubishi andNakajima were asked to build twoprototypes each, a further proposal fromKawasaki was rejected. The Mitsubishi design was an all-metal mid-wingcantilevermonoplane with retractablelanding gear, ventralbomb bay and tworadial engines.[3] 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.[4] In the resulting competition, Mitsubishi's Ki-21 and Nakajima'sKi-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 theNakajima 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".[5]
Production aircraft began to enter service in August 1938, supplementing and then replacing theFiat BR.20 bombers which had been purchased as an interim measure.[6]
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.[7]
TheKi-21-Ia was used in combat in thewar with China by the 60thSentai from autumn 1938, carrying out long-range unescorted bombing missions in conjunction with the BR.20 equipped 12th and 98thSentais. The Ki-21 proved to be more successful than the BR.20, having a longer range and being more robust and reliable. Two moreSentais, 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 theNomonhan Incident in June–July 1939.[6]
Losses were high during early combat operations, with weaknesses including a lack of armament andself-sealing fuel tanks, while the aircraft's oxygen system also proved unreliable. TheKi-21-Ib was an improved version designed to address the armament issue by increasing the number of 7.7 mm (0.303 in)Type 89 machine guns to five, and incorporating improvements to the horizontal tail surfaces andtrailing edgeflaps. In addition, the bomb bay was enlarged. The tail gun was a 'stinger' installation and was remotely controlled. Also, thefuel tanks were partially protected with laminated rubber sheets.[8][9]
This was followed in production by theKi-21-Ic with provision for a 500 L (130 US gal) auxiliary fuel tank, fitted in the rear weapons bay and one more 7.7 mm (0.303 in) machine gun, bringing the total to six. Four 50 kg (110 lb) bombs were carried externally. To offset the increase in weight, the main wheels of the Ki-21-IC were increased in size.[10][9]
However, by theattack on Pearl Harbor and the start of thePacific War, improvements in theROC Air Force caused losses to mount, and most Ki-21-1a, -1b and -1c were relegated to training or second-line duties.
Front line units from mid-1940 were equipped with theKi-21-IIa ("Army Type 97 Heavy Bomber Model 2A") with the more powerful 1,118 kW (1,500 hp)Mitsubishi Ha-101 air-cooled engines and larger horizontal tail surfaces. This became the main version operated by most IJAAF heavy bomber squadrons at the beginning of the Pacific War and played a major role in many early campaigns. For operations over thePhilippines, the JAAF's 5th, 14th and 62nd Air Groups, based inTaiwan, attacked American targets atAparri,Tuguegarao,Vigan and other targets inLuzon on 8 December 1941. The 3rd, 12th, 60th, and 98th Air Groups, based in French Indochina, struck British and Australian targets inThailand andMalaya, bombingAlor Star,Sungai Petani andButterworth under escort byNakajima Ki-27 andKi-43 fighters. However, starting from operations overBurma in December 1941 and early 1942, the Ki-21 began to suffer heavy casualties fromCurtiss P-40s andHawker Hurricanes.
To partially compensate, the IJAAF introduced theKi-21-IIb, with a pedal-operatedupper turret with one 12.7 mm (0.50 in)Type 1 machine gun, redesigned cockpitcanopies and increased fuel capacity. Although used in all fronts in the Pacific theater, it became clear by 1942 that the design was rapidly becoming obsolete, and was increasingly shifted away from front-line service.
In spite of its shortcomings, the Ki-21 remained in service until the end of the war, being utilized as transport (along with the civil transport versionMC-21), bomber crew andparatroopertrainer,glider towing,liaison and communications, special commando and secret missions, andkamikaze operations.
Nine Ki-21-Ia/b's were sold by Japan to Thailand in 1940 for use by theRoyal Thai Air Force againstVichy French forces inFrench Indochina but did not participate in theFrench-Thai War, as its crews had not completed training.[11]
Towards the end of the war, remaining Ki-21s were used byGiretsu Special Forces in strikes against American forces inOkinawa and theRyūkyū Islands. One of the noted operations was anattack on the Allied-heldYontan airfield andKadena airfield on the night of 24 May 1945. Twelve Ki-21-IIb's of theDaisan DokuritsuHikōtai were dispatched for a strike, each with 14commandos. Five managed to crash-land on the Yontan airfield. Only one plane landed successfully. The surviving raiders, armed withsubmachine guns and explosives, then wrought havoc on the supplies and nearby aircraft, destroyed 70,000 US gallons (260,000 L) of fuel and nine aircraft, damaging twenty-six more.[12]
A number of Ki-21-Ia were modified to serve as military transports for use byGreater 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 ofMitsubishi Ki-57, with equivalent civil aircraft being designated MC-20.
The Ki-21 had more than one Allied codename. Initially called "Jane", the name was quickly changed to "Sally" when GeneralDouglas 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.[13]
Data fromThe Concise Guide to Axis Aircraft of World War II[16]
General characteristics
Performance
Armament
Related development
Aircraft of comparable role, configuration, and era
Related lists
