Warplanes of Japan: Mitsubishi Ki-21

Mitsubishi Ki-21

Mitsubishi Ki-21-Ia (Army Type 97 Heavy Bomber Model 1A), codename “Sally”.

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 the Second WorldWar. It began operations during the Second Sino-Japanese War participating inthe Nomonhan Incident, and in the first stages of the Pacific War, includingthe Malayan, Burmese, Dutch East Indies and New Guinea Campaigns. It was alsoused to attack targets as far-flung as western China, India and northernAustralia. The Allies designated it under the reporting names "Sally"/"Gwen".

In 1936, the Imperial Japanese Army Air Service issued a requirement for a newheavy 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, topspeed of 400 km/h (250 mph), endurance of at least five hours, and a bombloadof 750 kg (1,650 lb). The design parameters were very ambitious for themid-1930's; few twin-engine bombers anywhere in the world could exceed suchperformance at that time.

Both Mitsubishi and Nakajima were asked to build two prototypes each, a furtherproposal from Kawasaki was rejected. The Mitsubishi design was an all-metalmid-wing cantilever monoplane with retractable landing gear, ventral bomb bayand two radial engines.[3] The first prototype flew on 18 December 1936, withthe second prototype, which differed in replacing the dorsal turret of thefirst prototype with a long greenhouse canopy, following later in the month.[4]In the resulting competition, Mitsubishi's Ki-21 and Nakajima's Ki-19 werefound to be similar, with the Ki-21 having better performance, while theNakajima design was a better bombing platform and had more reliable engines. Inorder to make a final decision, two further prototypes were ordered from bothMitsubishi 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 bythe Nakajima design and vice versa, while the Ki-21 gained a revised glazednose similar to that of the Ki-19 and revised tail surfaces. Thus modified, theKi-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 andthen replacing the Fiat BR.20 bombers which had been purchased as an interimmeasure. 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.

The Ki-21-Ia was used in combat in the war with China by the 60th Sentai fromautumn 1938, carrying out long-range unescorted bombing missions in conjunctionwith the BR.20 equipped 12th and 98th Sentais. The Ki-21 proved to be moresuccessful than the BR.20, having a longer range and being more robust andreliable. Two more Sentais, the 58th and 61st deployed to Manchuria in thesummer of 1939 for operations against China, with aircraft from the 61st alsobeing heavily used against Russian and Mongolian Forces during the NomonhanIncident in June–July 1939.

Losses were high during early combat operations, with weaknesses including alack of armament and self-sealing fuel tanks, while the aircraft's oxygensystem also proved unreliable. The Ki-21-Ib was an improved version designed toaddress the armament issue by increasing the number of 7.7 mm (0.303 in) Type89 machine guns to five, and incorporating improvements to the horizontal tailsurfaces and trailing edge flaps. In addition, the bomb bay was enlarged. Thetail gun was a 'stinger' installation and was remotely controlled. Also, thefuel tanks were partially protected with laminated rubber sheets.

This was followed in production by the Ki-21-Ic with provision for a 500 L (130US 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) bombswere carried externally. To offset the increase in weight, the main wheels ofthe Ki-21-IC were increased in size. However, by the attack on Pearl Harbor and the start of the Pacific War,improvements in the ROC 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 the Ki-21-IIa ("ArmyType 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. Thisbecame the main version operated by most IJAAF heavy bomber squadrons at thebeginning of the Pacific War and played a major role in many early campaigns.For operations over the Philippines, the JAAF's 5th, 14th and 62nd Air Groups,based in Taiwan, attacked American targets at Aparri, Tuguegarao, Vigan andother targets in Luzon on 8 December 1941. The 3rd, 12th, 60th, and 98th AirGroups, based in French Indochina, struck British and Australian targets inThailand and Malaya, bombing Alor Star, Sungai Petani and Butterworth underescort by Nakajima Ki-27 and Ki-43 fighters. However, starting from operationsover Burma in December 1941 and early 1942, the Ki-21 began to suffer heavycasualties from Curtiss P-40s and Hawker Hurricanes.

To partially compensate, the IJAAF introduced the Ki-21-IIb, with apedal-operated upper turret with one 12.7 mm (0.50 in) Type 1 machine gun,redesigned cockpit canopies and increased fuel capacity. Although used in allfronts in the Pacific theater, it became clear by 1942 that the design wasrapidly becoming obsolete, and was increasingly shifted away from front-lineservice. In spite of its shortcomings, the Ki-21 remained in service until the end ofthe war, being utilized as transport (along with the civil transport versionMC-21), bomber crew and paratrooper trainer, glider towing, liaison andcommunications, special commando and secret missions, and kamikaze operations. Nine Ki-21-Ia/b's were sold by Japan to Thailand in 1940 for use by the RoyalThai Air Force against Vichy French forces in French Indochina but did notparticipate in the French-Thai War as its crews had not completed training.

Towards the end of the war, remaining Ki-21s were used by Giretsu SpecialForces in strikes against American forces in Okinawa and the Ryūkyū Islands.One of the noted operations was an attack on the Allied-held Yontan airfieldand Kadena airfield on the night of 24 May 1945. Twelve Ki-21-IIb's of theDaisan Dokuritsu Hikōtai were dispatched for a strike, each with 14 commandos.Five managed to crash-land on the Yontan airfield. Only one plane landedsuccessfully. The surviving raiders, armed with submachine guns and explosives,then wrought havoc on the supplies and nearby aircraft, destroyed 70,000 USgallons (260,000 L) of fuel and nine aircraft, and damaged 26 more.

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 Chinafor transportation. Designated "MC-21", these aircraft had allarmament and military equipment removed. Used primarily as cargo transports,each could also seat nine paratroopers. Aircraft built from the start astransports were given the separate designation of Mitsubishi Ki-57, withequivalent 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 DouglasMacArthur objected that the name was the same as that of his wife. When theKi-21-IIb entered service, the absence of the long dorsal"greenhouse" led Allied observers to mistake it for a completely newtype, which was designated "Gwen". However, when it was realized that"Gwen" was a new version of the Ki-21, it was renamed "Sally3", with "Sally 1" referring to the earlier Ha-5 powered models,and "Sally 2" referring to the Ha-101 powered Ki-21-IIa. (Wikipedia)