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. 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. The second prototype arrived and flew at Muroc in August. On 7 September 1946, set a new U.S. National Speed Record-611 mph.

The National Air & Space Museum holds the nose section of the record setting second XP-84 (#476). The original cockpit canopy was destroyed during testing and has since been replaced. The cockpit section was transferred to the NASM by what is now the USAF Museum in Dayton, Ohio.

The Republic Thunderjet/Thunderstreak/Thunderflash family of jet-powered fighter-bombers and reconnaissance planes was one of the most important postwar combat aircraft. This plane equipped many Allied and NATO air forces until the advent of supersonic aircraft.

Republic Aviation Corporation's XP-84 design proposal was submitted in response to a General Operation Requirement (GOR) issued in September 1944. Republic's design group, headed by Alexander Kartveli, had also spearheaded the design of the Seversky P-35 and the Republic P-47. The original Thunderjet proposal was an attempt to fit a General Electric TC-180 axial-flow engine into a modified P-47J. The idea was a failure and was scrapped. In November 1944, Republic's revised design became the XP-84 upon acceptance by the Army Air Forces. They settled on a cantilever low-wing monoplane with straight, laminar-flow wings and a cantilevered horizontal stabilizer mounted halfway up the vertical tail. Only three XP-84 aircraft were ordered. The first two were powered by GE's J35-GE-7 producing 3,750 lb. of thrust, featuring axial flow. These engines consumed less fuel than the centrifugal-flow engines of earlier jet fighters such as the Lockheed P-80 Shooting Star.

The first flight of the prototype XP-84, serial no. 45-59475, occurred at Muroc Dry Lake in California, on 28 February 1946. Major William A. Lien piloted the aircraft as the first new American fighter to make its maiden flight after the end of World War II. In tests, it 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. The second prototype arrived and flew at Muroc in August. On 7 September 1946, the XP-84 set a new U.S. National Speed Record of 611 mph. The third aircraft was improved with the addition of an Allison J-35-A-15 producing 4,000 lb. of thrust, and re-designated the plane XP-84A, the only Thunderjet so designated.

The XP-84 featured a very clean design compared to other jet aircraft of the time. The P-59, for example, featured low-slung appendages, comprising intakes, engine bays, and exhausts. The P-80 had bulging intakes on each side to feed its centrifugal flow TG-160 engine. The British jets had either twin engine-pods or twin booms. The Russian jets still tended to look pot-bellied. The Me-262, as clean as a twin-pod engine arrangement could be made, was still not as sleek as the Thunderjet. The smaller diameter of the axial-flow engine allowed the use of a more streamlined, low drag fuselage. The intake for the jet engine was built into the nose. The pressurized cockpit was protected by a teardrop canopy and was equipped with an ejection seat.

The XP-84, as the prototype, had several unique features. None had auxiliary tanks, their navigation lights were midpoint at the extreme wing tips, and the pitot tube was at midpoint on the leading edge of the left wing. As with the P-80, painting was thought to help airflow, and thus the first XP-84 was painted gray. In practice, paint was of no benefit, and subsequent aircraft were simply buffed smooth and they flew in their coat of aluminum. A number of NATO aircraft were camouflaged for combat.

Because range as well as high speed was an important consideration, a thin-wing-profile wing gave way to 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 limitation to performance on early P-84 models.

In mid-1948, the newly independent USAF re-designated all pursuit (P) aircraft as fighters (F). Consequently re-designated as the F-84, the Thunderjet went on to serve with distinction during the Korean War. Its basic airframe gained a new lease on life in 1950, when a sweptwing version, initially designated the XF-96A and later F-84F, was developed and put into production under the name of Thunderstreak.

More exotic spin-offs also emanated from the original F-84 design: the turbine-propeller-driven, T-tailed XF-84H, unofficially called the "Thunderscreech," and the jet-and-rocket-powered XF-91 Thunderceptor, whose sweptback wings featured a unique inverse taper (being thicker and wider outboard from the wing root). In December 1951, an XF-91 became the first American airplane to exceed the speed of sound in level flight, but by that time simpler, more conventional aircraft would clearly be capable of the same feat soon, and only two prototypes of the Thunderceptor were built.

The National Air & Space Museum holds the nose section of the record setting second XP-84. The original cockpit canopy was destroyed during testing and has since been replaced. The cockpit section was transferred to NASM by what is now the USAF Museum in Dayton, Ohio.