After initiating the unlucky P-43 Lancer project, Alexander Kartveli, the head Republic designer, began work in 1939 on two other fighters derived from the Lancer. They were models AP-4 and AP-l0. The AP-4 followed in the line of the P-35 and was powered by a radial engine, while the AP-l0 was designed around the liquid-cooled Allison V-12 engine as a small, light fighter. Paradoxically enough, the massive P-47 was derived from the lightweight AP-10. Indeed, when Kartveli submitted the AP-l0 design to USAAC authorities on August 1, 1939, it was rejected and he was asked to develop a larger and more powerful version. In November the company signed a contract for two prototypes (the XP-47 and the XP-47A) to be powered by liquid-cooled Allison engines. The Allison engines turned out to be a mistake. The first military experiences in Europe had made it clear that planes must be better armed and better armored and give higher performance as well. The Allison engine was not powerful enough, nor could it provide satisfactory performance at high altitude. Therefore Kartveli developed an alternate project in terms of the most powerful engine then available, the new 2,000-h.p. Pratt & Whitney Double Wasp radial. He literally designed his plane around this large engine and its complex exhaust-gas-powered feeding system. He submitted it to USAAC authorities in June, 1940, and this time the project, the XP-47B, was accepted without reservation. A first order for 773 planes, at a value of $56.5 million, was placed while the plane was still on the drawing board. Of these, 170 were built as P-47Bs, 602 as P-47Cs, and one as the XP-47E, with a pressurized cabin.

The prototype XP-47B took to the air on May 6, 1941, after the finishing touches had been added, especially to the engine and feed system. During flight tests the Thunderbolt showed what it could do, reaching speeds over 410 m.p.h., with an ascent time of five minutes to 15,000 feet. It achieved these results with a takeoff weight of almost five and a half tons. In March, 1942, the first production models started coming oft the assembly line, and in January, 1943, the 56th Fighter Group was the first to make the new fighter operational, with the 8th Air Force based in Great Britain.

Production really hit its stride with the P-47D, which was the largest series. The P-47D had a more powerful engine at high altitude and a heavier bomb load. These Thunderbolts were built in several production lots and with a host of designations. Beginning with the P-47D-25, the planes incorporated a substantial modification, which had been introduced in the P-51 Mustang as well: a transparent teardrop hood that provided the pilot with 360 degrees visibility. The D model was the first to go to the Pacific in USAAF units and was the first to be supplied to the Soviet Union, Great Britain, Brazil, Mexico, and to Free French units.