To meet an Imperial Japanese Army specification of December 1937 for a ground-attack aircraft, which it was suggested could be a development of the Ki-30 light bomber. Great emphasis was placed on manoeuvrability, protection for the crew and the capability of operating from emergency airfields located near the combat area. Specifications called for a maximum speed of no less than 260 mph (420 km/h) at 6,578 ft (2000 m), take-off weight was to be 5,960 lbs (2700 kg) and it was to have a bombload of at least 440 lbs (200 kg) and defensive armament consisting of three machine guns, one which was on a moveable mounting. Mitsubishi produced two prototypes under the designation Mitsubishi Ki-51 in the summer of 1939. Of similar external appearance to the Ki-30, the new design was generally of smaller dimensions, had a revised and simplified cockpit that put the two-man crew more closely together and, because the bomb bay was not required, the monoplane wing was moved from a mid to low-wing configuration. Powerplant chosen was the Mitsubishi 940 hp (701 kW) Ha-26-II radial engine.

Tested during the summer of 1939, the two prototypes were followed by 11 service trials aircraft, these being completed before the end of the year. They differed from the prototypes by incorporating a number of modifications, but most important were the introduction of fixed leading-edge slots to improve slow-speed handling and armour plate beneath the engine and crew positions. Ordered into production in this form as the Army Type 99 Assault Plane, the Ki-51 began a production run that totalled 2,385 aircraft, built by Mitsubishi (1,472) and by the First Army Air Arsenal at Tachikawa (913), before production ended in July 1945. 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 realisation 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. Subsequently, three Ki-71 tactical reconnaissance prototypes were developed from the Ki-51, introducing the 1,500 hp (1119 kW) Mitsubishi Ha-112-II engine, retractable landing gear, two wing mounted 20 mm cannon and other refinements, but no production examples were built.

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.

Variants

Mitsubishi Ki-51a - A single Ki-51 conversion resulted in the Ki-51a tactical reconnaissance prototype. Never put into production.

Mitsubishi Ki-71 - Mitsubishi designed and Tachikawa arsenal built three prototypes of a dedicated tactical reconnaissance aircraft powered by the 1,500 hp (1119 kW) Ha-112-II engine and equipped with retractable landing gear. Never put into production. 

(Army Type 99 Assault Plane - Mitsubishi Ki-51)

Allied Codename: Sonia

Type: Two Seat Ground Attack & Reconnaissance

Design: Mitsubishi Jukogyo KK Design Team (Kawano, Ohki and Mizuno, who had designed the Ki-30)

Manufacturer: Mitsubishi Jukogyo KK (1,472) & Tachikawa Dai-Ichi Rikugun Kokusho (1st Army Air Arsenal - 913)

Powerplant: One 1,500 hp (1119 kW) Mitsubishi Ha-26-II radial engine.

Performance: Maximum speed 264 mph (425 km/h) at 9,845 ft (3000 m); service ceiling 27,130 ft (8270 m).
Range: 659 miles (1060 km) on internal fuel.

Weight: Empty 4,129 lbs (1875 kg) with a maximum take-off weight of 6,437 lbs (2920 kg).

Dimensions: Span 39 ft 8 1/4 in (12.10 m); length 30 ft 2 1/4 in (9.20 m); height 8 ft 11 1/2 in (2.73 m); wing area 258.56 sq ft
(24.02 sq m).

Armament: Two fixed forward firing 7.7 mm (0.303 in) Type 97 machine guns, one 7.7 mm (0.303 in) Type 92 machine gun on a trainable mount in rear cockpit plus a bombload of up to 441 lbs (200 kg). On late model aircraft the two fixed 7.7 mm guns were replaced by 12.7 mm (0.50 in) machine guns.
Variants: Ki-51a (single prototype), Ki-71.

Avionics: None.

History: First flight summer 1939; first flight (Ki-71) 1941; production ended with Japan's surrender.