The Republic F-84 Thunderjet was an American-built turbojet fighter-bomber aircraft. Originating as a 1944 United States Army Air Force proposal for a "day fighter", the F-84 flew in 1946. Although it entered service in 1947, the Thunderjet was plagued by so many structural and engine problems that a 1948 Air Force review declared it unable to execute any aspect of its intended mission and considered cancelling the program. The aircraft was not considered fully operational until the 1949 F-84D model and the design matured only with the definitive F-84G introduced in 1951.