XP-75

In February of 1942, the USAAF issued a Request For Proposals for a fighter/interceptor of high performance. Maximum speed was to be 440 mph at 2000 feet, operational ceiling was to be 38,000 feet, and range was to be 2500 miles. A special requirement was added for the initial climb rate, which was to be no less than 5600 feet per minute.

In April of 1942, Donovan Berlin (who had been responsible for the P-36 and the P-40) left the Curtiss company to take over the directorship of the Aircraft Development Division of the Fisher Body Division of the General Motors Corporation. One of his first assignments was in response to the USAAF request. In September of 1942, Fisher submitted their proposal using the most powerful liquid-cooled engine then available, the 2,885-hp twenty-four cylinder Allison V-3420. This engine was basically a pair of coupled V-1710 engines, mounted side-by-side in a W-type configuration. Significant savings in cost and time were to be gained by employing major assemblies from existing aircraft already in production in the manufacture of the new interceptor.

The first of two prototype XP-75s that flew on 17 November 1943 was built with P-51 Mustang outer panels, F4U Corsair landing gear and A-24 Dauntless empennage.

On October 10, 1942, a contract for two prototypes was awarded to Fisher under the designation XP-75.

Serial numbers 43-46950 and 43-46951 were assigned to the two XP-75 prototypes. The Allison engine was to be located behind the cockpit and was to drive a set of contra-rotating propellers via an extension shaft and a reduction gearbox. The engine was cooled by a duct in the ventral fuselage. Initially, it was planned that the outer wing panels of the P-51 Mustang would be used in an inverted-gull configuration. However, at an early stage it was decided to drop the inverted gull-wing configuration and go with a straight wing design utilizing outer wing panels from a P-40.

By the summer of 1943, the USAAF had a more urgent need for long-range escort fighters than it did for fast-climbing interceptors. The urgent demand to exploit existing components was abruptly dropped. On July 6, 1943, the USAAF ordered six more prototypes that would be adapted to fulfill the long-range escort role. They were assigned the designation XP-75A, and the serial numbers were 44-32161/32166. They were to be powered by an Allison V-3420-23 engine, and were to be armed with six 0.50-inch machine guns in the wings and four 0.50-inch guns in the fuselage nose. At the same time, the USAAF decided to order 2500 production P-75As, although they did stipulate in the contract that if the production aircraft did not meet specifications the order might be cancelled. Maximum speed was to be 434 mph at 20,000 feet and 389 mph at sea level. These production P-75As were to be built at the Fisher plant in Cleveland, Ohio.

P-75A Eagle

The first XP-75 flew on November 17, 1943. It was powered by an Allison V-3420-19 engine rated at 2600 hp for takeoff and driving a pair of contra rotating propellers. As planned, the wing was of a straight center section with P-40 outer panels with modified tips. The tail assembly was from an A-24, and the main undercarriage members were taken from an F4U Corsair.

The ‘hybrid’ test ships were fol-lowed by six production XP-75As with bubble canopy rather than the braced hood of earlier machines and a modified tail assembly. The first of these flew on 15 Septem-ber 1944 and had most compo-nents designed new from the out-set. It featured the V-3420-23 engine of the XP-75A.

All six XP-75A long-range escort versions were in the test program by the spring of 1944. Some problems were encountered with instability, since errors had been made in the initial estimate of the aircraft's center of mass. The coupled Allison engine failed to give its full rated power. The engine cooling was inadequate, aileron forces were excessively high, and the spinning characteristics were poor. Though it could reach a speed of 400 mph, the XP-75 was disappointing in almost every other aspect.

At that stage in the war, the Republic P-47D Thunderbolt and the North American P-51D Mustang were more than adequately fulfilling the long-range escort role, and the USAAF decided that there was no longer any need for a new escort fighter.

Consequently, the USAAF decided to terminate the P-75 development program, and the production contract for the P-75A was cancelled on October 27, 1944. The serial numbers of the six P-75As built were 44-44549/44553.

By the time of contract termination, the first and second P-75A had been delivered to Elgin Field, Florida for tactical suitability trials, the third machine was in the shop being fitted with an experimental intercooler, and the fourth and fifth machines were almost complete.

Although the USAAF no longer had any need for the P-75, it was decided to go ahead and finish these machines and use them for development work. The sixth machine was to be placed in storage and scavenged for spare parts to keep the rest flying.

The five production P-75As never completed official performance trials, but enough testing was performed to confirm the fact that the maximum speed was at least 30 mph below that guaranteed by the manufacturer. The third machine received an experimental intercooler installation which permitted substantial increases in engine power.

The improved version was still unsatisfactory and after three Eagles had crashed, the entire program was cancelled on November 8, 1944. Only eight XP-75s and six P-75As were built.

The last production P-75A (serial number 44-44553) was on display in the Annex at the Wright-Patterson Air Force Base Museum in Dayton, Ohio, in 1992. This was apparently the aircraft that had been scavenged to keep the other P-75s flying, so more than a few things may be missing. The P-75A on display was loaned to the USAF Museum by the National Air and Space Museum of the Smithsonian Institution in 1977.

XP-75

Engine: Allison V-3420-23, 2885 hp

Span: 49 ft. 4 in.

Length: 40 ft. 5 in.

Height: 15 ft. 6 in.

Wing area: 347 sq.ft

Empty weight: 11,495 lb

Normal loaded weight: 13,807 lb

MTOW: 18,210 lb

Maximum speed: 404 mph.

Max speed: 433 mph at 20,000 ft

Cruising speed: 250 mph.

Range: 2,600 miles

Range max external fuel: 3500 miles

Service Ceiling: 29,500 ft.

Absolute ceiling: 36,400 ft.

Initial climb rate: 4200 fpm at 10,000 ft

Climb rate: 3900 fpm at 20,000 ft

Crew: One

Armament: six 0.50-inch machine guns wings (235 rpm) / four 0.50-in machine guns fuselage (300 rpm) / two 500-pound bombs