A swept-wing version of the F-84 had been planned late in 1949 in order to bring the performance of the Thunderjet up to the level of that of the F-86 Sabre. However, it was hoped that the swept wing version would retain or even improve the ground attack capabilities of the Thunderjet. At this time, USAF funds for new aircraft development were rather limited, and it was thought that by using some 60 percent of existing Thunderjet tooling and a standard F-84E fuselage, a high-performance aircraft could be produced at minimal cost.
The swept-wing version of the Thunderjet was initially designated XF-96A because of its extensive design changes. The USAF sanctioned construction of a single prototype. The last aircraft on the F-84E contract (49-2430) was selected for the conversion. The fuselage of the XF-96A was essentially that of the F-84E, including the pilot's rearward-sliding canopy and the air brake underneath the fuselage. The tail was swept back and an entirely new wing, swept back at an angle of 38.5 degrees was used. The new wing had an area of 325 square feet and a span of 36 feet 5 inches. The wing had its maximum thickness at 45 percent chord, although the chord-wise thickness ratio was only nine percent. The swept wing had an anhedral (negative dihedral) of 3.5 degrees. The engine was an Allison J35-A-25 of 5200 lb.st.
The construction of XF-96A 49-2430 took 167 days. It was then dismantled and flown from Farmingdale via cargo aircraft out to Muroc. It was reassembled there and took off on its maiden flight on June 3, 1950, piloted by Republic's Director of Flight Otto P. Haas. Performance included a maximum speed of 693 mph at sea level (Mach 0.93), and a range of 1716 miles at a cruising speed of 514 mph. Maximum fuel load was 1253 Imp. gall internally and externally, and the empty and maximum loaded weights were 12,150 pounds and 23,350 pounds respectively. Although the low-altitude speed was impressive, performance fell off rather badly with altitude. Service ceiling was only 38,300 feet and it took 14.8 minutes to reach an altitude of 35,000 feet. It was felt that the performance improvement over the F-84E was only marginal at best, and the project proceeded with only low priority.
Were it not for the Korean War, the XF-96A project probably would have been cancelled. As it turned out, the onset of the Korean War freed up some USAF funds for continued development and a letter contract for F-96A production was received in July of 1950. On August 9, 1950, the designation was changed to XF-84F, and the name Thunderstreak was chosen. It was anticipated that the first production Thunderstreaks would be available in the autumn of 1951.