The Kawasaki Ki-61 Hien (Swallow) was remarkable in being the only Japanese fighter to feature an inline engine, as a result it greatly resembled the P-51 and the Bf 109 in appearance. The Ki-61 was designed around the Daimler-Benz DB 601A (later produced domestically) engine which was first brought to Japan by U-Boat yet suffered from a number of reliability problems which were never truly solved throughout the aircraft's service life which involved extensive action over New Guinea, the Philippines, and later over Japan. In spite of this, the aircraft was well-liked thanks to its good protection and armament (yet another departure from Japanese convention) which was enough to take on B-29s in their own operating altitude as well as being a fairly good match against most of its expected opposition including early-war Allied fighter such as the P-40. A late-war development from surviving airframes resulted in the radial-engined Ki-100, considered by many to be one of Japan's finest fighters of the war and fully capable of meeting any US fighter on equal terms.

The Ki-61 began development in December 1940 and the production Ki-61-Ia and -Ib were first deployed in New Guinea during April 1943. Armament was inadequate, a problem which was addressed in the Ki-61-I-KAIc and -I-KAId with were equipped with 20-mm wing-mounted cannon. The final variant was the Ki-61-II with a more powerful Kawasaki Ha-140 engine which eventually incorporated a larger wing and rudder and was known as the Ki-61-II-KAI but which saw very limited service due to disruption by B-29 bombing raids. Surviving airframes were eventually converted into the superlative Ki-100 with radial Ha-112 engines. Its main variants were the Ki-100-Ia basic fighter and the Ki-100-Ib with an all-vision canopy. The turbocharged Ki-100-II was also drawn up but the Japanese surrender came before it ever had a chance to see combat.