Many countries used floatplanes for scouting and reconnaissance duties, and to hunt submarines and surface ships, but only Japan built and fielded fighters on floats intended to gain air superiority above a beachhead to support amphibious landing operations where carrier or land-based fighters were unavailable. The Kawanishi N1K1 (Allied codename REX) was the only airplane designed specifically for this purpose to fly during World War II.

Many countries used floatplanes for scouting and reconnaissance duties, and to hunt submarines and surface ships. But only Japan built and fielded fighters on floats intended to gain air superiority above a beachhead to support amphibious landing operations where carrier or land-based fighters were unavailable. The Kawanishi N1K1 (Allied codename REX) was the only airplane designed specifically for this purpose to fly during World War II.

In September 1940, the Japanese Navy issued a specification for floatplane fighters capable of supporting offensive naval operations. A team of engineers including Toshihara Baba, Shizuo Kikuhara, Hiroyuki Inoue, and Elizaburo Adachi had readied the first prototype by May 1942, and it flew on May 6. The team's original design used a pair of contra-rotating propellers to offset the powerful torque effects of the 1,460 horsepower Mitsubishi engine, but the struggled with the propeller gearbox functioning properly. Kawanishi reverted to one three-blade propeller and service trials with the revised aircraft resumed in August 1942.

Tests showed that the airplane was structurally very sound. Speed was only slightly less than the Mitsubishi A6M Zero (see NASM collection) and the amphibious fighter was almost as maneuverable. This was remarkable performance for an aircraft that could not retract or jettison its huge landing gear.

The Navy ordered the Kyofu into production and the first operational models appeared in the spring of 1943. However, by this time, Japan was no longer on the offensive and a high-performance fighter-class floatplane was not needed. Navy strategists stopped the N1K1 production line and the last REX was completed in March 1944. A total of 89 production aircraft and 8 prototypes preceded it.

The combat career of these aircraft was unremarkable. The first Kyofu unit formed up in early 1944 at Balikpapen, Borneo. When the unit disbanded less than two months later, it claimed 29 Allied airplanes confirmed destroyed and seven probably destroyed for the loss of five REXs. These claims are probably exaggerated.

Other than minor liaison work, the only other aerial combat that involved Kyofus happened over the home islands in 1945. A Kyofu detachment operated from Lake Biwa, northwest of Kyoto on the main Japanese island of Honshu, but details about this unit's operation history remain unknown.

The National Air and Space Museum N1K1 is one of just two Kyufus extant. The other example is displayed at the Admiral Nimitz Center in Fredericksburg, Texas. The NASM aircraft probably never flew in an operational unit. It was one of 145 Japanese aircraft brought to the United States after the war for evaluation but unfortunately, no test records survive. In the late 1940s, the U. S. Navy moved the airplane to Naval Air Station Norfolk, Virginia, along with other aircraft awaiting transfer to the Smithsonian. The REX arrived in 1972 and Smithsonian officials placed it in outdoor storage at the Silver Hill Facility (now the Paul Garber Facility) until suitable indoor storage space became available in 1975.