To meet an Imperial Japanese Army specification of December 1937 for a two-seat ground-attack aircraft, which it was suggested could be a development of the Ki-30 light bomber, Mitsubishi produced two prototypes under the designation Mitsubishi Ki-51.
The Ki-51 was a cantilever low-wing monoplane, with a 'trousered' and spatted non-retractable main landing gear and a fixed tail wheel. The powerplant for both prototype and production aircraft was a single 940-hp Mitsubishi Ha-26-II 14-cylinder air-cooled radial engine, driving a three-blade variable-pitch metal propeller. The green-house canopy was shorter than that on the Ki-30, bringing the two-man crew closer together, and a degree of armour protection was provided for the occupants and the engine. The Ki-51 differed from the Ki-30 in having no internal bomb bay, the normal 200-kg (440-lb) weapon load being carried externally. Overall construction was of metal except for the control surfaces, which were fabric-covered.
Two prototypes, designated Ki-51, were completed in June and August 1939, followed immediately by a pre-series batch of 11 more, all completed by the end of that year. They differed from the prototypes by incorporating a number of modifications, including the introduction of fixed leading-edge slots to improve slow-speed handling and armour plate beneath the engine and crew positions.
After a virtually problem-free flight test programme, and only a few minor modifica-tions, the Ki-51 entered production in January 1940 as the Army Type 99 Assault Aircraft. From then until March 1944 Mit-subishi's Nagoya factory manufactured a total of 1459. Four months later a second production line was established by Tachikawa, which had completed a further 913 by July 1945. Standard armament at first consisted of three 7.7-mm (0.303-in) Type 89 machine-guns, one in each wing and one in a movable mounting in the rear cockpit. On later aircraft, the wing-mounted pair were exchanged for Type 1 guns of 12.7-mm (0.5-in) calibre, and the aircraft's range was increased by installing a 68-litre (15-Imp gal) fuel tank in each wing leading edge. The Ki--51 served with at least eight Sentais (groups) and a dozen or more independent squadrons of the Japanese army air force, from 1940 until the end of the Second World War. It established a reputation for being reliable and capable of surviving considerable battle dam-age, which made it popular with its crews. It was given the Pacific codename Sonia by the Allies. Adapted, inevitably, for the kamikaze role towards the end of the war, it was flown as a single-seater, carrying a 250-kg (550-lb) bomb beneath the fuselage.
In addition to the standard production aircraft, there were attempts to develop dedicated reconnaissance versions, initially by the conversion of one Ki-51 service trials aircraft which had the rear cockpit redesigned to accommodate reconnaissance cameras. Test and evaluation of this aircraft, redesignated Ki-51a, brought a realization that the standard Ki-51 could be modified to have provisions for the installation of reconnaissance cameras, and this change was made on the production line.
Three prototypes were completed at Tachikawa of the Ki-71 (codenamed Edna), an improved, retractable-gear version with a 1500-hp / 1119kW Mitsubishi Ha-112-11 engine and two 20mm wing cannon, but no production orders for this type were placed.
Allocated the Allied codename 'Sonia', the Ki-51 was used initially in operations against China, and was deployed against the Allies until the end of the Pacific war. In more intensely contested areas the fairly slow Ki-51s were easy prey for Allied fighters, but in secondary theatres, where an ability to operate from rough and short fields was valuable, these aircraft gave essential close support in countless operations. In the closing stages of the war they were used in kamikaze attacks.
Engine: 940-hp / 705kW Mitsubishi Ha-26-II radial
Prop: three-blade variable-pitch metal
Wingspan: 12.1 m / 39 ft 8 in
Length: 9.21 m / 30 ft 3 in
Height: 2.73 m / 8 ft 11 in
Wing area: 24.02 sq.m / 258.55 sq ft
Max take-off weight: 2798-2920 kg / 6169 - 6438 lb
Empty weight: 1873 kg / 4129 lb
Max. speed: 424 km/h / 263 mph
Ceiling: 8270 m / 27150 ft
Range: 1060 km / 659 miles
Armament: 3 x 7.7mm machine-guns, 200-300kg of bombs
Engine: 1500-hp / 1119kW Mitsubishi Ha-112-11
Armament: two 20mm wing cannon