The Republic Thunderjet/Thunderstreak/Thunderflash family of jet-powered fighter-bombers and reconnaissance planes was one of the most important of postwar combat aircraft, and equipped many allied and NATO air forces until the advent of supersonic aircraft.
The series had its origin in a 1944 company-financed design study for a jet-powered replacement for the famed P-47 Thunderbolt. At first, Alexander Kartveli and his team at the Republic Aircraft Corporation considered a straightforward jet adaptation of the P-47 airframe, but soon decided that such a design was impractical and began over again from scratch. They settled on a cantilever low-wing monoplane with straight, laminar-flow wings and cantilevered horizontal tailplanes mounted halfway up the vertical tail. A large airbrake was to be installed in the belly of the aircraft, just underneath the cockpit. The engine selected was the General Electric TG-180 (J35) turbojet. This engine had an axial flow, which offered less fuel consumption than that of the centrifugal-flow engines of earlier jet fighters such as the Lockheed P-80 Shooting Star. The smaller diameter of the axial-flow engine had the additional advantage in that it allowed the use of a more streamlined, low drag fuselage. The intake for the jet engine was to be mounted in the nose. The pressurized cockpit was to have a teardrop canopy and was to be equipped with an ejector seat.
Since range as well as high speed was an important consideration, it was necessary to forego a thin profile wing in favor of an airfoil section that was thick enough to carry fuel tanks and landing gear. The critical Mach number of this wing was considerably lower than that of the fuselage, and was the primary limiter of performance on early P-84 models.
The USAAF liked what they saw, and ordered three prototypes and 400 production examples in March of 1945. The designation P-84 was chosen. However, with the coming of victory in the Pacific, all existing military aircraft orders were suspended pending a review of postwar needs. Most outstanding orders were cancelled in their entirety, but on January 15, 1946 the USAAF confirmed its order for 15 YP-84A service test aircraft and 85 production P-84Bs.
The first XP-84 (serial number 45-59475) was completed in December of 1945, and was powered by a 3750 lb.st. General Electric J35-GE-7 turbojet. It was partially disassembled and flown from the factory at Farmingdale, New York to Muroc AFB in California aboard the Boeing XC-97 transport. The XP-84 took off on its maiden flight on February 28, 1946, piloted by Major William A. Lien. It was the first new American fighter to have its maiden flight after the end of World War II.
In tests, the XP-84 achieved a maximum speed of 592 mph at sea level. Normal range was 1300 miles, and an altitude of 35,000 feet could be attained in 13 minutes. Weights were 9080 pounds empty, 13,400 pounds gross, and 16,200 pounds maximum.
The second XP-84 flew in August 1946. On September 7, 1946, this XP-84 set a new American speed record of 611 mph. However, that very same day, the record was snatched away by the Gloster Meteor, which reached 616 mph.