The Kawasaki Ki 61 Hien Swallow (Tony) went into production in August 1942 as the Army Type 3 Fighter Model 1 Hien but did not see action until June 1943. Initial tests showed it to be faster and more maneuverable than all other Japanese fighter aircraft except the KI-43 (Oscar).
While most Japanese fighter aircraft were designed for maneuverability the KI-61 was designed entirely for speed and power. It was such a radical departure from all other Japanese fighter aircraft that the Allies first assumed that it was not a Japanese design but either a German or Italian designed aircraft. When it was first encountered it was give the code name of Antonio (Tony) by the Allies.
It was powered by an inverted 12 cylinder water cooled engine built under license from Daimler Benz. This was the only water cooled fighter aircraft built by the Japanese. It was fast, light weight, and highly maneuverable. The Hien had four 50 cal (12.7 mm) machine guns (2 in the engine cowling and 2 in the wings) and some later models were equipped with the Mauser MG-151 20mm automatic cannon in the wings and then with the Japanese Ho-50 automatic cannon when it went into production
The KI61-2n had an operational range of 1120 miles which could be increased by the addition of two 53 gallon drop tanks.
When the Akashi plant that was producing the 12 cylinder Ha-140 engine was destroyed by B-29 bombers in January 1945 the airframe was modified to accept the Mitsubishi Ha-112-II 14-cylinder twin row air cooled radial engine which produced 1500 HP. This new version was designated the KI-100.
The KI 61 was the first Japanese fighter aircraft designed with self sealing fuel tanks and pilot armor, making it a much more durable fighter and able to take quite a bit of punishment and still be effective in battle.
3,3159 Ki 61 type I, type II, and KI-100 fighters were produced during the war.