Republic XP-47B Thunderbolt

Last revised June 5, 1999

The Republic P-47 Thunderbolt originated from the drawing board of Alexander Kartveli of the Seversky Aircraft Corporation (later renamed Republic Aviation). The Thunderbolt is consistently rated as one of the three outstanding USAAF fighters of World War II-- rated right up there along with the North American P-51 Mustang and the Lockheed P-38 Lightning. The P-47 was built in larger numbers than any other American fighter, 15,683 examples rolling off the assembly line before production finally ended.

At one time during the heady days of 1944, there were no less than 31 front-line fighter groups flying Thunderbolts. Thunderbolts fought on all fronts in World War 2, including Alaska. Approximately two-thirds of all Thunderbolts built actually reached operational units overseas. In two and a half years of combat, from March 1943 to August 1945, these Thunderbolts flew over half a million combat missions, destroying over 12,000 enemy aircraft both in the air and on the ground, as against a total of 5222 Thunderbolts lost, only 824 of them in the heat of combat. This corresponded to 54 percent of the Thunderbolts which went overseas being eventually lost either to enemy action or to accidents, which was a fairly typical attrition rate for a wartime fighter. Losses of Thunderbolts on operational missions were 0.7 percent of those dispatched, an exceptionally low figure.

By the end of the war, the Thunderbolt had established an overall ratio of air-to-air combat victories to losses of 4.6 to 1. Thunderbolts dropped 132,482 tons of bombs, fired 59,567 rockets, and expended 135 million belts of machine gun ammunition.

From D-Day to V-E Day in Europe, Thunderbolts destroyed 86,000 railway cars, 9000 locomotives, 6000 armored vehicles and tanks, and 68,000 trucks. By the end of the war, Thunderbolts had destroyed 2752 enemy aircraft in the air and 3315 on the ground.

The P-47 as originally conceived was quite different from the aircraft which was ultimately to emerge from the Republic factories. On August 1, 1939, Kartveli, in response to an official requirement, proposed a lightweight high-altitude interceptor to the USAAC under the company designation of AP-10. It was to be powered by a 1150 hp Allison V-1710-39 liquid-cooled in-line engine. This was a radical change in design philosophy for Kartveli, since he had always preferred radial air-cooled engines for fighters because of their greater simplicity and their ability to absorb a larger amount of battle damage. Gross weight was to be 4900 pounds and estimated maximum speed was 415 mph. Armament was to be a pair of 0.50-in machine guns mounted in the engine housing.

The USAAC looked over the proposal and was favorably impressed. However, they deemed that additional armament would be required, even if it adversely affected performance. Kartveli increased the size of his AP-10 design somewhat, and added four wing-mounted 0.30-inch machine guns. Gross weight rose to 6570 pounds. In this guise, in November 1939 the USAAC ordered one prototype of the AP-10 design under the designation XP-47. The serial number was 40-3051.

In addition, on January 17, 1940, the USAAC ordered a stripped, unarmed version of the same basic design under the designation XP-47A. It was to be devoid of armament, radio and other tactical equipment so that it could be tested before the fully-equipped XP-47. The serial number of the XP-47A was 40-3052.

In the meantime, combat reports coming in from Europe were changing everyone's ideas about air combat. More firepower, more armament, more armor protection, and self-sealing fuel tanks were likely to be required in future air battles. Both the XP-47 and XP-47A had insufficient engine power to accommodate the additional weight required by these features, and the USAAC came to the conclusion that these designs were likely to fall far short of future air combat requirements. The Army considered the XP-47 to be insufficiently armed, and thought that it had too high a wing loading and was too slow in comparison with the Curtiss XP-46. Anticipating that the Army would ultimately reject his XP-47 design, Kartveli went back to the drawing board.

In order to accommodate the heavy firepower, armor, and self-sealing fuel tanks and still provide a performance capable of meeting enemy aircraft on equal terms, a lot of engine horsepower would be needed. Kartveli decided to produce a design based around a turbosupercharged Pratt and Whitney R-2800 Twin Wasp eighteen-cylinder air-cooled radial engine, one of the most powerful aircraft engines available at the time. Armament was to be a set of eight 0.50-in machine guns in the wings, following the RAF's trend toward eight-gun fighters and making the Republic proposal among the heaviest-armed fighters yet considered by the USAAC up to that time. Total weight was to be a massive 11,500 pounds, unprecedented for a USAAC single-seat fighter. A maximum speed of 400 mph at 25,000 feet and 340 mph at 5000 feet was envisaged. It was anticipated that an altitude of 15,000 feet could be reached in five minutes.

On June 12, 1940, Kartveli submitted his ideas to the USAAC. The USAAC was sufficiently impressed with the proposal that on September 6, 1940 ordered a prototype under the designation XP-47B. This designation was sort of unusual at the time, namely, using the same P-number for what was in effect a totally new design. All work on the XP-47 and the XP-47A was cancelled, and the serial number of the abortive XP-47 was transferred to the XP-47B.

One week later, on September 13, 1940, 773 production examples of the new fighter were ordered by the USAAC, 171 to be delivered as P-47Bs and 602 as P-47Cs. At the same time, the Army contract placed back in 1939 for 80 P-44 Rockets was cancelled. The contract was replaced with an order for a similar quantity of P-43 Lancers which would keep the Farmingdale production lines occupied pending the introduction of the new fighter.

Kartveli decided to design the XP-47B fuselage around the large turbosupercharger from the start, rather than to add it onto the aircraft later as sort of an afterthought. In order to preserve a streamlined fuselage with a small cross-section, the large turbosupercharger was placed in the rear fuselage. It was fed by an air duct located beneath the large R-2800 engine. Engine exhaust gases were directed back to the rear fuselage in separate pipes to the turbine and were expelled through an exhaust under the rear tail. Ducted air was fed to a centrifugal impeller and was returned to the engine under pressure via an intercooler.

Another problem that had to be solved was that the aircraft required a very large twelve-foot diameter four-bladed propeller in order to take full advantage of the R-2800 engine's high power output. This large propeller in turn required a long and stalky undercarriage in order that the propeller be given adequate ground clearance during takeoff and landing. If a conventional retractable undercarriage were used for the P-47, its suspension would have to have been placed very far outboard on the wings, leaving insufficient space for the eight wing guns and their ammunition. In order to solve this problem, the landing gear telescoped and was nine inches shorter when retracted than when extended. Somewhat surprisingly, this complex telescoping landing gear seems to have caused few problems in the field.

Like the earlier P-35 and P-43 fighters, the P-47 was a cantilever low-wing monoplane, the wing being elliptical in shape with the ailerons on the outer trailing edge and flaps on the inner trailing edge. The semi-monocoque fuselage was all metal, but initially the control surfaces were fabric covered. The tailwheel was steerable and was fully retractable. All the fuel tanks were inside the fuselage and were self-sealing from the start. The cockpit was protected by armor and was unpressurized.

The name *Thunderbolt* for the P-47B was originally thought up by C. Hart Miller, Republic's Director of Military Contracts. The company approved his choice, and the name stuck.

The XP-47B prototype (40-3051) flew for the first time on May 6, 1941, piloted by Lowry L. Brabham. This was only eight months after the order had been placed. The XP-47B was the largest single-engine fighter built up to that time. At a loaded weight of 12,086 pounds, the XP-47B dwarfed all previous fighters, being almost twice as heavy as most of its contemporaries. On the first flight, the pilot was forced to make an unplanned emergency landing because of a leakage of exhaust fumes into the cockpit. Its eighteen-cylinder XR-2800-21 radial engine offered 1960 hp at 25,800 feet, and gave it a maximum speed of 412 mph, 12 mph faster than Kartveli had projected. An altitude of 15,000 feet could be attained in five minutes. Empty and normal gross weights were 9189 pounds and 12,086 pounds respectively. The prototype was destroyed in an accident on August 8, 1942.


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