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General characteristics
Crew: 1
Length: 8.94 m (29 ft 4 in)
Wingspan: 12 m (39 ft 4 in)
Height: 3.7 m (12 ft 2 in)
Wing area: 20 m2 (220 sq ft)
Airfoil: NACA 2R 16 wing root, NACA 24009 tip
Empty weight: 2,630 kg (5,798 lb)
Gross weight: 3,470 kg (7,650 lb)
Fuel capacity:
Internal 550 l (150 US gal; 120 imp gal)
External 2x 200 l (53 US gal; 44 imp gal)
Powerplant: 1 × Kawasaki Ha40 inverted liquid-cooled V-12 piston engine, 864 kW (1,159 hp)
Propellers: 3-bladed variable pitch propeller

Performance
Maximum speed: 580 km/h (360 mph; 313 kn) at 5,000 m (16,000 ft)
Range: 580 km (360 mi; 313 nmi)
Service ceiling: 11,600 m (38,100 ft)
Rate of climb: 15.2 m/s (2,990 ft/min)
Time to altitude: 7.0 min to 5,000 m (16,000 ft)
Wing loading: 173.5 kg/m2 (35.5 lb/sq ft)
Power/mass: 0.25 kW/kg (0.15 hp/lb)

Armament
Guns:
2× 20 mm (0.79 in) Ho-5 cannon, 120 rounds per gun each
2× 12.7 mm (0.50 in) Ho-103 machine guns, 250 rpg each
Bombs: 2× 250 kg (550 lb) bombs

The Kawasaki Ki-61 Hien (飛燕, “flying swallow”) was a Japanese Army aircraft designated as “Army Type 3 Fighter”. Initialy it was often mistaken by Allied pilots as a Messerschmitt Bf 109 or an Italian aircraft. This was most likely due to being the only mass-produced Japanese fighter of the war to use a liquid-cooled engine. Ironically enough, the aircraft did use a Kawasaki build licensed DB 601 (the same power plant used with the Bf-109), known as the Ha-40.

The Ki-61 was to be a light general-purpose fighter, intended to be used in offensive air superiority roles at low to medium altitudes. While seeing extensive service in China and Southeast Asia, the aircraft is probably most known for intercepting American Boeing B-29 Superfortresses over China and the Japanese homeland Islands. The tactic of using the aircraft to ram the enemy was not an uncommon practise during these operations. These attacks were executed to such an extent that ‘Special Attack Units’ were formed, which even included 244th Hiko Sentai (Fighter group). Even if the pilot survived such an attack, they remained obliged to continue these deadly and dangerous ramming tactics until they were killed or no longer able to fly.

The Hien entered combat in the spring of 1943 in the New Guinea war zone, covering New Guinea, the Admiralty Islands, New Britain, and New Ireland. The new Japanese fighter caused some pain and consternation among Allied pilots, particularly when they found out the hard way that they could no longer go into a dive and escape as they had from lighter Japanese fighters. 5th Air Force Commander General George Kenney found his P-40 Warhawks completely outclassed, and begged for more P-38 Lightnings to counter the threat of the new enemy fighter.

The Ki-61 demonstrated only a few teething problems in field use, such as a tendency towards engine overheating during ground running under tropical conditions. However, despite the heavier armament, it still didn't have the punch to easily knock rugged and well-armed Allied bombers out of the sky.

The Kawasaki designers had foreseen this problem. The Japanese Ho-5 20 millimeter cannon wasn't available at the time, but the Japanese obtained 800 Mauser MG 151/20 20-millimeter cannon from Germany in August 1943, and modified 388 Ki-61-I airframes on the production line to carry the German weapons in place of the two 12.7-millimeter wing guns. The cannon had to be mounted on their sides to fit into a wing, with an underwing blister for the breech, and some reinforcements were added to the wing to absorb the heavier recoil.

Once the Ho-5 cannon finally became available, Kawasaki designers then reversed the arrangement of the guns, putting the 20-millimeter cannon in the nose and the 12.7-millimeter guns in the wings. While they were making these modifications, they also made a few changes to streamline manufacturing and simplify field maintenance.

This new variant was designated the "Ki-61-I KAIc" (where "KAI" was for "kaizo", or "modified"). It was 19 centimeters (7.5 inches) longer than its predecessors, and also featured a detachable rear section; a fixed tailwheel instead of the earlier retractable tailwheel; stronger wings; and stores pylons outboard of the main landing gear, allowing it to carry two 250 kilogram (550 pound) bombs.

The Ki-61-I KAIc went into production in January 1944 and ultimately replaced production of all earlier models in August 1944. A few "Ki-61-I KAId" bomber interceptors were also built in late 1944. These aircraft featured two 12.7 millimeter guns in the fuselage and a 30 millimeter Ho-105 cannon in each wing. Total production of the Ki-61-I KAIc/d was 1,274 aircraft.

Very unlike it's other contemporary Japanese brothers, the Hien (or Swallow) had an inverted-Vee engine. In fact, it was a license built German Daimler-Benz DB 601A. This, and other features made the Allied pilots think it was maybe a license built Messerschmitt Bf 109, or an Italian aircraft. This resulted in the Allied codename 'Tony'.
A number of reasons resulted in the development of the radical different fighter in the Japanese arsenal. Kawasaki failed to receive contracts for their models with other types of liquid cooled engines, so work started on a license built version of a proven type. Also German engineers worked with the Kawasaki company to design and build fighters that would win a production contract. Furhtermore Kawasaki learned some facts of the airial warfare in the North European Theatre during 1939 - 1940. This resulted in self sealing fuel tanks and armor protection for the pilot.
Work on the Ki-61 Hien was designed in paallel with the Ki-60, a cannon-armed interceptor whose emphasis was more on speed and climb-rate than on agility and range. The Ki-61 was to be the multi-role stable-mate of the Ki-60, with an emphasis on agility and long range.
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