The Kawasaki Ki-61 was designed around a licence-built copy of the Messerschmitt 109 engine. It was faster the Allied fighters of the time, apart from the P-38. Many P-40 pilots found to their cost that they could no longer dive to escape trouble. It entered service with the Japanese Imperial Army in 1943, and although successful, it was difficult to maintain as the V12 engine was prone to overheating. Eventually there were no more engines and the Ki-61 mutated into the Ki-100 with it's much more robust radial engine.

The 'Hien' was armed with 2 x 12.7mm machine guns and 2 x 20mm cannon. Two racks could also fitted to carry bombs or fuel tanks.