The Kawasaki Ki-45 required more time to develop and place in service than almost every other Japanese warplane of World War II. Takeo Doi, chief project engineer, began work on this design in January 1938 but the first production aircraft did not fly combat until the fall of 1942. When it finally entered service, the Ki-45 soon became popular with flight crews who used it primarily for attacking ground targets and ships including U. S. Navy Patrol Torpedo (P. T.) boats. The Toryu was also the only Japanese Army night fighter to see action during the war.

Japanese strategists observed the Americans and the Europeans design and build a number of twin-engine, two-seat, heavy fighters during the mid- and late 1930s. The Japanese Army needed a long-range fighter to cover great distances during any large-scale conflict in the Pacific and army planners felt that a twin-engine design could meet this need. In March 1937, the Japanese Army Staff sent a rather vague specification for such an airplane to a number of manufacturers. Kawasaki, Nakajima, and Mitsubishi responded, but the latter two dropped out of the competition to concentrate on other projects. Between October and December 1937, the army amended the specification with additional information and directed Kawasaki to begin the design work. The specification described a two-seat fighter with a speed of 540 kph (336 mph), an operating altitude of 2-5,000 m (6,560-16,405 ft), and endurance of over 5 hours. The army chose the Bristol Mercury engine, built under license, to power the new aircraft.

In January 1939, Kawasaki rolled out the first prototype but initial flight tests did not impress. The airplane was too slow to meet the army speed requirement, and it suffered mechanical problems with the landing gear and engines. Top speed remained a problem, despite major changes on the second prototype, and the army put the project on hold. In April 1940, Kawasaki substituted 14-cylinder Nakajima engines, rated at 1000 horsepower each, for the original 9-cylinder motors rated at 820 horsepower each. Engineer Doi also revised the engine nacelles and prop spinners. These modifications increased top speed to 520 kph (323 mph) but the revisions continued. Kawasaki narrowed the fuselage, increased the wing span and area, revised the nacelles again, and modified the armament package. The new aircraft did not fly until May-June 1941 but performance at last met army standards and they ordered the Toryu into production.

Kawasaki delivered the first Ki-45 Kai (modified) in August 1942 but Toryus did not reach combat units in China until October. Unlike many Japanese Navy fighter airplanes, the Ki-45 aircraft had crew armour and fire-resistant fuel tanks. These airplanes also carried a heavy gun battery that usually consisted of 20 mm and 37 mm cannons. Toryus operated in the New Guinea area against Allied shipping and attacked Consolidated B-24 Liberator bombers of the 5th Air Force. The Japanese also employed some Ki-45s as night fighters. Field personnel modified these Toryus by substituting the upper fuselage fuel tank for two 12.7 mm machine guns mounted to fire obliquely upwards at a target's vulnerable belly. This worked so well that the army told Kawasaki to manufacture a night fighter version of the Toryu-the Ki-45 Kai (Mod. C)-with two 20 mm cannon, mounted obliquely, and a 37 mm cannon mounted in the lower fuselage.

In June 1944, 20th Air Force bomber crews flew Boeing B-29 Superfortresses on the first raids against the Japanese home islands since Doolittle's attack back in May 1942. Bad weather and attacks by Japanese fighter interceptors, including Ki-45 Toryus, hampered these raids. On one mission, Ki-45 pilots downed eight Superfortresses.

On March 9, 1945, the 20th Air Force began flying low altitude attacks at night using incendiary bombs. These missions marked a radical departure from the traditional American high-altitude, daylight bombing strikes. The Japanese fought back with anti-aircraft gunfire and night fighter attacks. As many as six Sentais (groups) of NICK night fighters defended the home islands by war's end. The Ki-45 Kai Hai (Mod. C) the Japanese Army's only night fighter, operated alongside Navy night fighters including the Nakajima J1N1-S Gekko (IRVING) and P1Y1-S Byakko (FRANCIS). Examples of the IRVING and FRANCIS are also preserved in NASM's collection. The NASM Ki-45 Kai Hai (Mod. C) is the last known survivor of 1,700 Ki-45s built by Kawasaki. The company built a total of 477 Kai Hai C night fighters.

The NASM airplane was produced in the second of three batches and the thrust-augmentation exhausts fitted to the engines to improve speed and reduce glare at night identify aircraft in this batch. This NICK was one of about 145 Japanese airplanes returned to the United States for evaluation after the war. The Navy shipped them to Norfolk, Virginia, aboard the escort carrier USS Barnes. On December 8, 1945, the Navy transferred the NICK to the U. S. Army Air Forces at Langley Field, Virginia. Personnel at Langley shipped the Ki-45 to the Air Depot at Middletown, Pennsylvania, for overhaul and flight test. During the next few months, the aircraft was extensively test-flown at Wright Field, Ohio, and Naval Air Station Anacostia in the District of Columbia. During the army's evaluation, pilots reported that NICK handled very poorly on the ground. They also did not like the cramped cockpit, excessive vibration, and the poor visibility. Takeoff distance, climb speed, flight characteristics, approach and landing, and manoeuvrability were all rated as good to excellent.