The Mitsubishi Ki-51 Type 99 Assault Plane (Allied codename: Sonia) was a military attack aircraft used during World War II by the Imperial Japanese Army Air Force. The first production model of the Ki-51 series was the Ki-51-I model which had a single Mitsubishi Ha-26 engine which was capable of propelling it at speeds of up to 423 kilometers per hour.[1] However, the top speed of the aircraft was affected by the non retractable landing gear.

The aircraft had an armament that consisted of two 7.7 machine guns in the wings (two 12.7mm Ho-3 Machine guns in late models) and a single 7.7 machine gun in the gunner position. A single 200 kilogram bomb was used for traditional ground attack sorties, though a 250 kilogram bomb was used for kamikaze attacks.

The total weight of the aircraft was around 1,870 kilogram, non combat loaded while the total length was around nine meters.[2] The service ceiling maxed out at around 8,200 meters with a range of 1,060 kilometers.


No variants of the Ki-51-I platform were ever created, though several prototypes for variants were made. One example was converted into a Ki-51a tactical reconnaissance aircraft. Mitsubishi also designed a dedicated reconnaissance variant under the Ki-71 designation. Limited to three prototypes built by Tachikawa, this version had retractable landing gear and a 1,500 hp (1,119 Kw) Mitsubishi Ha-112-II engine. Neither reconnaissance version was selected for production.[3]


Development and construction

A Ki-51 "Sonia" abandoned by the 1st Sentai in New Guinea

Designed to meet the requirement for a ground attack aircraft for the Imperial Japanese Army, two prototypes of the Ki-51, powered by a single Mitsubishi Ha-26-II engine, were tested in the Summer of 1939. The Ki-51 differed from the similar Mitsubishi Ki-30 Type 97 Light Bomber due to the new type's low set wing and revised cockpit.[N 1] The 11 service trials aircraft which followed the prototypes introduced armour protection for both the engine and the crew, together with aerodynamic improvements to improve the type's handling qualities at low speeds. Of the 2,385 examples built, 1,472 were built by Mitsubishi, with the remaining 913 built by the Tachikawa 1st Army Air Arsenal.[3]

Operational Service

Following it's introduction in China, the Ki-51 served throughout the Pacific War, mainly in secondary theatres due to the type's vulnerability to Allied fighters, before being used for Kamikaze attacks.[3] However, despite its overall low speed, the plane was maneuverable and well armored so that even when the plane did encounter Allied aircraft, it had good survivability. Furthermore, just like its German counterpart, the Junkers Ju 87 Stuka, the Ki-51 had the ability to be used on very rough and ill-prepared airfields, perfect for the horrid conditions of the Pacific.

Camouflaged IJAAF Mitsubishi Ki-51 "Sonia" pictured at an unidentified airfield, circa 1943-1945[4]

Because of its merits, the Ki-51 was used until the end of the war, even in its intended roles of close support and reconnaissance. In fact, a couple of these aircraft claimed a few USN submarines and ended up getting the last kills by the Japanese of World War II. In total, approximately 2,300 examples had been produced by the end of the war.


  1. The Ki-30 had a mid set wing to allow the fitting of an internal bomb bay, which was not required for the smaller Ki-51.[3]


  3. 3.0 3.1 3.2 3.3 World Aircraft Information Files Aviation Partwork. Midsummer Books Ltd. File 901 Sheet 34 (A_Z of Aircraft: Mitsubishi Ki-51 (Type 99 Assault Plane) to Mitsubishi MU-2, Marquise and Solitare)
  4. U.S. Navy National Museum of Naval Aviation (Photo No. 1996.488.159.057) via Wikimedia

Community content is available under CC-BY-SA unless otherwise noted.