Kawasaki Ki-61 (Tony) Type 3 Fighter Hein
The Hein ("Swallow" in Japanese) was the only JAAF fighter with a inverted "V" liquid cooled engine. The Ki-61 prototype was completed in December 1941, and flew well, reaching a speed of 368 mph.  During the first half of 1942  the prototype was extensively tested, performing very well against a captured P-40E Warhawk and a German Messerschmidt Bf-109E sent to Japan by submarine. The Tony was the first Japanese fighter to enter production after the start of the war in the Pacific, and the only inline engined fighter produced.

Production
A total of 3,078 were built. All Ki-61s were assembled at the Kagamigahara Plant. Most of the Ki-100s were assembled at the Kagamigahara Plant, as well. Twelve Ki-100-Ib aircraft were assembled at the company's Miyakonojo Plant.

First Combat - Dolittle Raid Intercept
Interestingly, its first combat was over Japan when a test model flown by Lt. Umekawa happed to be flying on April 18, 1942 during the Dolittle Raid. He attacked a B-25 but had to break off due to lack of fuel and problems with his guns. On the B-25s crew reported the sighting, and this was mistakenly interpreted as evidence that the Japanese were importing or producing copies of the Messerchmidt 109.

Liquid Cooled Engine
Most Japanese Army and Navy aircraft were equipped with radial engines, while the Tony was one of the first with a liquid cooled engine. In 1937 Kawasaki had purchased a licence to build the German Daimler Benz DB.601 engine, used in the Messerschmitt 109.  Kawasaki revised and lightened the engine, creating their own new design in 1940 the Ha-40, inverted V-12 engine.  Around this engine Kawasaki planned the Ki-60 fighter, the production design being a lighter aircraft designated the Ki-61. 

Mistaken For Messerchmidt Bf-109
Initially in combat, the Tony was thought to be a German Messerchmidt Bf-109, due to its similar siloette and inline engine. Later, Allied intelligence learned that this type was an entirely new Japan design.

Into Combat
As the A6M Zero had done for the Navy, the Ki-61 did for the Army.  It could out run, dive and climb every Allied fighter in 1943 except the P-38 Lightning.  It was the first fighters to be equipped with “fire-proof jacket" for fuel tanks, (early form of self-sealing fuel tanks (inferior to Allied, but offered someprotection) and armor plate behind the pilot's head. It was rugged and able to withstand more damage than other Army fighter. But, it was plagued with mechanical and engine problems when flying in the tropics, and also a shortage of supplies and trained ground crews to maintain it. Many were lost in the air or on the ground in New Guinea. During home defense, the aircraft were up-gunned and used to intercept B-29s and for Taiatari (body crashing) attacks of raming American bombers.

Mauser 20mm Cannon In Tony
The Japanese army imported 800 Mauser 20mm cannon and ammunition for use in 400 Ki-61s. To distinguish the planes equipped with the Mauser cannon, Kawasaki and the army identified these planes by assigning four-digit MNs made by adding the base number of 3000 to each plane's true serial number: 3001-3400. The Japanese did not manufacture any cannon based upon these Mauser cannon. All of the Mauser cannon in Ki-61s were the German-made imports. Apparently no spares were initially on hand, but as planes were written off the cannon were removed and used as spares. Some reports say that 388 instead of 400 Ki-61s carried the Mauser cannon and that some of those were converted from earlier Ki-61s by field modification, but I haven't found that to be so.