• 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.

    On the day Hiroshima was destroyed by an atomic bomb, two Ki-51s were responsible for the last Japanese sinking of a US warship, sinking USS Bullhead (SS-332) with all hands.
    Charles Lindbergh, flying a P-38 Lightning, shot down a Ki-51.
    New Guinea

    Specifications (Ki-51)
    General characteristics
    Crew: 2
    Length: 9.21 m (30 ft 3 in)
    Wingspan: 12.1 m (39 ft 8 in)
    Height: 2.73 m (8 ft 11 in)
    Wing area: 24 m2 (260 sq ft)
    Empty weight: 1,873 kg (4,129 lb)
    Gross weight: 2,798 kg (6,169 lb)
    Max takeoff weight: 2,920 kg (6,437 lb)
    Powerplant: 1 × Mitsubishi Ha-26-II 14-cylinder air-cooled radial piston engine, 710 kW (950 hp)
    Propellers: 3-bladed variable-pitch propeller
    Maximum speed: 424 km/h (263 mph, 229 kn) at 3,000 m (9,843 ft)
    Range: 1,060 km (660 mi, 570 nmi)
    Service ceiling: 8,270 m (27,130 ft)
    Time to altitude: 5,000 m (16,404 ft) in 9 minutes 55 seconds
    Wing loading: 117 kg/m2 (24 lb/sq ft)
    Power/mass: 0.24 kW/kg (0 hp/lb)
    2× fixed, forward-firing 7.7 mm (.303 in) Type 89 machine guns (replaced with 2× 12.7 mm (.5 in) Ho-103 machine guns in later models)
    1× 7.7 mm (.303 in) Te-4 Machine gun rearward-firing machine gun.

    Bombs: 200 kg (441 lb) bombs (normal operations); 250 kg (551 lb) for suicide operations

Moderator(s): Boelcke, Buhli, cheruskerarmin, Cpt_Farrel, Duggy, Graf, Gumpy, Hayate, HBPencil, HEERDT, Jarink, Jaypack44, Juri_JS, kristorf, mapal, MarcoPegase44, monguse, PatCartier, PIPS, RAF_Loke, Rudi_Jaeger, Tailhook, Tomi_099, US_Grant