Originally designed to meet an Air Corps specification for a high-altitude point-defense interceptor, forced to become a long-range escort fighter for lack of an alternative, then finding its metier as a low-altitude ground attack aircraft, the amenability of the P-47 Thunderbolt to take on roles unforeseen when the XP-47B rolled out of Republic aircraft’s Farmingdale factory in early 1941 guaranteed its place in history as one of the truly outstanding aircraft of the World War II.

The P-47 was the final result of the work of one of the most visionary figures in aviation, Alexander P. deSeversky. Seversky’s most important contribution to aviation development was hiring fellow Georgian Alexander Kartveli. Born Alexander Kartvelishvili in Tblisi, Georgia, on September 9, 1896, he moved to Paris in 1914 to study aeronautical engineering and graduated in 1922 from the Highest School of Aviation in Paris. Between 1922–1927, he worked as a designer at the Louis Blériot company. In 1924, one of his designs established a world speed record. American millionaire Charles Levine invited him to come to New York, to join the Atlantic Aircraft Corporation in 1928, where he didn’t like working on Fokker designs. Kartvelishvili met Alexander deSeversky in 1931 and went to work for his fellow Georgian, “Americanizing” his name to Kartveli.

In late 1939, Kartveli commenced work on the AP-10, a lightweight fighter powered by the Allison V-1710 liquid-cooled V-12 engine; the eight .50-calbre M2 Browning machine guns were an astounding armament considering that the Spitfire and Hurricane each carried eight ,30-calibre weapons. The Air Corps backed the project, designating it XP-47.

By the spring of 1940, Kartveli and the Air Corps concluded that both the XP-44, the XP-47 and the XP-47A offered as a ‘fix’ were inferior to Luftwaffe fighters. Kartveli then designed a much larger fighter, offered to the USAAC in June 1940. Kartveli had learned his lesson from the USAAC rejection of the ‘wet wing’ and the new design was specifically a point-defense interceptor. The Air Corps and ordered a prototype in September, designated XP-47B, though it had nothing in common with either the XP-47 or the XP-47A.

The XP-47B was all-metal construction with fabric-covered elevators and rudder, with a semi-elliptical wing that went back to the P-35. The air-conditioned cockpit was roomy - so much so that when the airplane arrived in England, Spitfire pilots joked that when attacked, the pilot could unstrap and take evasive action dodging about the cockpit. Main and auxiliary self-sealing fuel tanks under the cockpit provided 305 U.S. gal, deemed more than sufficient for the intended interceptor role.

The powerplant was the new Pratt & Whitney R-2800 Double Wasp two-row 18-cylinder radial engine producing 2,000hp, using a four-bladed Curtiss Electric constant-speed propeller 12 feet 2 inches in diameter. The engine cowling ducted cooling air for the engine, left and right oil coolers, and the turbo supercharger intercooler system with exhaust gases routed into a pair of wastegate-equipped pipes running to either side of the cockpit providing power for the turbosupercharger located in the lower rear fuselage. At full power, the pipes glowed red at their forward ends, spinning the turbine at 21300rpm. The complicated ductwork and turbo supercharger gave the XP-47B a deep fuselage; thus, the wings were mounted relatively high, which in turn required long-legged main landing gear struts to provide ground clearance for the enormous propeller. In order to assure sufficient room in the wings for the guns and ammo, the gear legs telescoped the strut 9 inches when extended. The armament of eight Browning AN/M2 .50-calibre machine guns were mounted outboard of the landing gear, with their 500rpg ammunition bays filling the outer wing.

All of this meant the XP-47B was very heavy when compared to contemporaries. The empty weight of 9900lb was 65% more than the P-43; it would be the heaviest single-seat fighter used during the war. Kartveli said, "It will be a dinosaur, but it will be a dinosaur with good proportions.”

Republic test pilot Lowry P. Brabham first flew the XP-47B on May 6, 1941. By then it had been given the emotive name “Thunderbolt” and was impressive in early trials. The prototype was lost in an accident on August 8, 1942; by then it had achieved a level speed of 412mph at 25800ft and climbed from sea level to 15000ft in five minutes. While the XP-47B gave the newly renamed United States Army Air Forces cause for optimism, it had its share of teething problems that caused an equal amount of apprehension.

The Thunderbolt’s sheer size and limited ground-propeller clearance after lifting off the tail wheel into a fuselage-level attitude made for challenging takeoffs requiring long runways and meant the pilot had to hold the tail low until close to takeoff speed on the initial run. The sideways-opening canopy had a tendency to jam. The gun installation’s tight fit and cramped ammunition belt tracks led to jamming, especially during and after hard manoeuvering. The ailerons "snatched and froze" at high altitude while control loads were deemed excessive at higher speeds. Power was affected by ignition wiring arcing. Republic soon fitted a rearwards-sliding canopy, a pressurized ignition system and eventually all-metal control surfaces appeared in the combat-capable P-47C.

The USAAF ordered 171 P-47Bs shortly after the prototype’s first flight, with the first production P-47B delivered in December 1941.

Teething problems continued. A Republic test pilot was killed in the fifth production P-47B when it went out of control in a dive on March 26, 1942, crashing due to tail assembly failure after fabric-covered tail surfaces ballooned and ruptured. This was the first indication the Thunderbolt’s performance brought pilots into the transonic flight regime where they experienced the never-encountered phenomenon of “compressibility” as speed approached Mach 1.

The newly-formed 56th Fighter Group had moved to Mitchell Field, Long Island, not far from the Republic factory, in April 1942, and was the initial unit to equip with the P-47B, with the first arriving in June 1942. Their proximity to the factory made the 56th the operational evaluation unit for the new fighter over the next six months. As they gained experience over the course of the summer, Republic made numerous modifications as a result of unsatisfactory reports from 56th pilots and engineers.

In September 1942, the 56th group welcomed a new commander, Major Hubert Zemke. Born in Montana in 1914, Zemke had never exhibited any interest in airplanes or flying as a youth. Promoted to Captain, Zemke was sent to England in September 1940 as a combat observer with the Royal Air Force, where he studied the tactics of both the RAF and the Luftwaffe. The next year, he was sent to the Soviet Union to instruct Russian pilots on flying Lend-Lease P-40s. After his return to the United States in February 1942, he was one of the first Air Corps pilots to fly the XP-47B that spring. His experience with the P-47 and knowledge of European air combat made him a natural choice to take over the 56th.