The Mitsubishi Ki-21 'Sally' was the Japanese Imperial Army's most important heavy bomber during the Sino-Japanese War and for most of the Second World War. One of the best aircraft of its type when it first entered service, by 1944 it was almost obsolete, and had only remained in service for so long because of the lack of a suitable replacement.
On 15 February 1936 the Japanese Army issued a specification for a new heavy bomber to replace the Army Type 92 Heavy Bomber (Mitsubishi Ki-20) and Army Type 93 Heavy Bomber (Mitsubishi Ki-1). The new specification called for a twin-engined monoplane with a maximum speed of at least 248.5mph at 9,845ft, an operating attitude of 9,845-13,125ft, a clime rate of 8 minutes to 9,845ft, an endurance of over five hours at 186mph, a take-off run of less than 948ft, a bomb load of 1,653lb will full fuel and 2,205lb for short range missions.
Defensive armament was to consist of three flexibly mounted machine guns, and the aircraft was to be powered by either the 850hp Nakajima Ha-5 or Mitsubishi Ha-6. The aircraft was to carry a crew of four with space for two more. Recognising the likelihood of clashes with Russia on the border of Manchuria the aircraft had to be able to operate in extreme cold weather.
Three companies produced designs - the Mitsubishi Ki-21, Nakajima Ki-19 and Kawasaki Ki-22. Mitsubishi and Nakajima were both awarded contracts to produce prototypes, and the first prototype of the Ki-21 made its maiden flight on 18 December 1936. This aircraft was powered by the Mitsubishi Ha-6 engine, had an angular glazed nose, a stepped rear fuselage and a smaller tail than the eventual production aircraft.
The two designs underwent a series of competitive flight tests at Tachikawa between March and May 1937, and bombing tests at Hamamatsu in June. These revealed the Ki-21 to be slightly faster, and with a lighter wing loading, but the Ki-19 to be nicer to fly and a more stable bombing platform. Neither aircraft was completely satisfactory, so the Imperial Japanese Army ordered both companies to produce modified designs.
The third Ki-21 prototype, produced in response to this order, was powered by the Nakajima Ha-5 engine. It had a new hemispherical nose, with a 7.7mm Type 89 machine gun mounted on a ball-and-socket mount. The ventral step was removing, giving the aircraft a smoother rear fuselage. The rudder was modified in shape and made larger, improving the aircraft's directional stability.
The two designs competed in a second set of service trials at Tachikawa, and this time the Ki-21 was a clear winner. It was ordered into production as the Army Type 97 Heavy Bomber Model 1. The urgent need for new aircraft after the start of the Sino-Japanese War in 1937 meant that Nakajima were also given an order to produce the Ki-21. Deliveries began in August 1938, and the first unit to convert to the Ki-21 was the 60th Sentai, which used them in the fighting over China.
A total of 2,064 Ki-21s were produced. Mitsubishi produced 431 Ki-21-Is and 1,282 Ki-22-IIs, while Nakajima produced 351 Ki-21-Is.
The Ki-21 was an all-metal cantilever mid-wing monoplane. The wings had a narrow straight edged centre-section and tapering outer sections that sloped upwards. The oval fuselage was of stressed-skin construction, with the bomb bay under the wing centre section. The prototype has an angular glazed nose, but that was replaced on production aircraft by a smoother hemispherical nose, containing the bombardier and a flexibly mounted machine gun. The first prototype had a semi-hemispherical dorsal turret. On the Ki-21-I this was replaced by a long greenhouse, before a similar turret was restored on the Ki-21-II.
At first the Ki-21 was give the codename 'Joan', after MacArthur's wife. When it was discovered that the general did not approve of this the code name was changed to 'Sally'.
At first the Ki-21-IIb was considered to be a new aircraft, and so was given the codename 'Gwen'. When the Allies learnt that is was actually a version of the Ki-21 that was changed to 'Sally 3'. The earlier Ha-5 powered aircraft became 'Sally 1' and the Ha-101 powered versions 'Sally 2'.
The Ki-21-Ia entered front line service with the 60th Sentai, in the second half of 1938, operating over China from bases in Manchuria. It was not a great success. Although it was more popular with its crews than the Italian Fiat B.R.20s it replaced, it carried half the bomb load. The long distances in China meant that the Ki-21 was forced to operate without fighter escorts, and its poor armament and lack of protection meant that the 60th Sentai and 61st Sentai both suffered heavy losses. These losses began to fall after the -Ia was replaced by the more heavily armed -Ib and -Ic, but most of this was due to the destruction of the Chinese Air Force, rather than any improvement in the survivability of the Ki-21.
The Ki-21-IIa was the main type in use by December 1941. The Japanese Imperial Army was allocated the task of invading Malaya and Burma, while also continuing the war in China. The 3rd Hikoshidan (Air Division) was moved from China to French Indo-China to support the invasion of Malaya. Its Ki-21-IIa met with early successes over Malaya, where the main Allied fighter was the Brewster Buffalo. The Ki-21-IIa also performed well over Hong Kong, where there were no Allied fighter aircraft at all.
By the end of December it had become clear that these early successes were misleading. On 20 December the Ki-21 clashed with P-40s of the American Volunteer Group over Kumming, while on 23 December they faced a mix of RAF Buffaloes from No.67 Squadron and aircraft from the A.V.G. during raids on Rangoon and Mingaladon. On 23 December twenty Ki-21s were shot down at a cost of two Allied pilots. Only a shortage of Allied fighter aircraft and the chaotic situation on the ground saved the Ki-21s from continued heavy losses. The same was true during the Japanese conquest of the Dutch East Indies, where the Ki-21 performed well when facing obsolete or disorganised opposition, but was vulnerable whenever Allied fighters were encountered in larger numbers.
Despite suffered from increasingly heavy losses the lack of a suitable replacement meant that the Ki-21 remained in front line service in large numbers until well into 1944. It was forced to operate against increasingly powerful Allied fighter forces in New Guinea and over Burma, and suffered correspondingly heavy losses. The Ki-21 was also the main bomber used during the series of Japanese bombing raids on Calcutta in December 1943, where once again it suffered heavily.
After the arrival of the Mitsubishi Ki-67 Hiryu the Ki-21 was rapidly withdrawn from front line service, and by the end of the war only the 58th Sentai was still using the type. A number were used for suicide attacks, while the aircraft's last great success came on 24 May 1945, when nine Ki-21s filled with commandos attempted to land on Okinawa. Only one aircraft reached its target, landing on Yontan airfield, where the commandos destroyed seven American aircraft and burnt 2,600 drums of petrol.
Ki-21-Ia (Army Type 97 Heavy Bomber Model 1A)
The Ki-21-Ia was the first production version of the aircraft. It was similar to the prototype, but with extra fuel capacity, carrying 580 Imperial Gallons, up from 405. It was powered by the Army Type 97 radial engine, the production version of the Nakajima Ha-5.
Mitsubishi built 143 Ki-21-Ias. Nakajima produced 351 Ki-21-1s, but the number of each variant is apparently unknown.
Engine: Two x Nakajima Ha-5 Kai
Power: 950hp at take-off, 1,080hp at 13,125ft
Wing span: 73ft 9 13/14in
Length: 52ft 5 29/32in
Height: 14ft 3 13/32in
Empty Weight: 10,342lb
Loaded Weight: 16,517lb
Maximum Weight: 17,452lb
Max Speed: 268mph at 13,125ft
Service Ceiling: 28,215
Climb to 16,405ft in 13min 55sec
Range: 932 miles (normal), 1,680 miles (maximum)
Armament: Three 7.7mm machine guns
Bomb-load: 2,205lb (maximum)
Ki-21-Ib (Army Type 97 Heavy Bomber Model 1B)
The Ki-21-Ib was produced in an attempt to increase the defensive firepower of the Ki-21. Two more 7.7mm guns were added to the original nose, dorsal and ventral guns of the Ki-21-Ia were retained. One was a remote controlled 'stinger' gun carried in the tip of the tail, while the second was a flexibly mounted gun that could fire out of lateral openings on either side of the rear fuselage. The Ki-21-Ib also had a larger bomb bay, laminated rubber sheets to protect the fuel tanks, larger landing flaps and larger horizontal tail surfaces. Mitsubishi produced 120 Ki-21-Ibs, and Nakajima an unknown number.
Ki-21-Ic (Army Type 97 Heavy Bomber Model 1C)
The Ki-21-Ic saw another increase in defensive firepower, carrying a second lateral machine gun. It could also carry a 110 gallon auxiliary fuel tank in the rear bomb-bay, and four 110lb bombs under the wings (to be used alongside the auxiliary fuel tank). It also had larger main wheels, needed to absorb the increased weight of the aircraft. Mitsubishi produced 160 Ki-21-Ics, delivering the last in December 1940. Nakajima produced an unknown number, delivering their last aircraft in February 1941. This ended Nakajima's involvement with Ki-21 production, as they began to concentrate on their own unsuccessful Ki-49.
Ki-21-II (Army Type 97 Heavy Bomber Model 2)
In November 1939 Mitsubishi were ordered to produce an improved version of the Ki-21, with better performance and a higher service ceiling. The first production Ki-21-Ic became the prototype for the -II. It was given two 1,500hp Mitsubishi Ha-101 14-cylinder air-cooled radial engines, a larger engine that required a larger engine nacelle and allowed Mitsubishi to give the aircraft a fully retractable undercarriage. The horizontal tail surfaces were also increased in size. Flight trials began in March 1940, and the aircraft was ordered into production as the Ki-21-IIa (Army Type 97 Heavy Bomber Model 2A). Four service-test aircraft were produced before the first production aircraft appeared.
Ki-21-IIa (Army Type 97 Heavy Bomber Model 2A)
The Ki-21-IIa was the production version of the Ki-21-II. It was similar to the prototype, but with some minor equipment changes. A total of 590 Ki-21-IIas were built between December 1940 and 1942, and it soon replaced the older Ki-21-I in front line units.
Ki-21-IIb (Army Type 97 Heavy Bomber Model 2B)
The Ki-21-IIb was produced after the failure of the Nakajima Ki-49. The only significant change from the Ki-21-IIa was the removal of the greenhouse with its flexibly mounted 7.7mm gun and its replacement with a manually operated conical turret armed with a single 12.7mm machine gun. The turret was rotated used bicycle pedals attacked to a chain drive, while the gun was flexibly mounted. 688 Ki-21-IIbs were produced by Mitsubishi between 1942 and September 1944. Production finally ended when the Mitsubishi Ki-67 Hiryu provided the Imperial Japanese Army with a suitable replacement.
Engine: Two x Mitsubishi Ha-101 14-cylinder radial engine
Power: 1,500hp at take-off, 1,340hp at 15,090ft
Wing span: 73ft 9in
Length: 52ft 6in
Height: 15ft 11in
Empty Weight: 13,382lb
Loaded Weight: 21,407lb
Maximum Weight: 23,391lb
Max Speed: 302mph at 19,685ft
Cruising Speed: 236mph at 16,405ft
Service Ceiling: 32,810ft
Range: 1,678 miles (normal)
Armament: Five 7.7mm machine guns, in nose, ventral, tail and two beam positions and one 12.7mm machine gun in dorsal turret
Bomb-load: 2,205lb (1,000kg)