In February of 1942, the USAAF issued a Request For Proposals (RFP) which called for aircraft companies to submit designs for a fighter/interceptor having an exceptional 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. High speed, high climb rate, high ceiling, and long range, all in one package--a tall order for any aircraft company.
In April of 1942, the famous designer Donovan Berlin (who had been responsible for such successful designs as the P-36 and the P-40) left the Curtiss company after many years of service to take over the directorship of the Aircraft Development Division of the Fisher Body Division of the General Motors Corporation. As one of his first assignments, he took it upon himself to work on a design in response to the USAAF RFP. In September of 1942, Fisher submitted their proposal to the USAAF. The Fisher proposal used the most powerful liquid-cooled engine then available, the 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.
On October 10, 1942, a contract for two prototypes was awarded to Fisher under the designation XP-75. In assigning the XP-75 designation to the Fisher design, the designations XP-73 and XP-74 were skipped, for reasons which are not altogether clear even today. The aviation historian James Fahey claims that the P-73 and P-74 designations were deliberately omitted as a result of political pressure applied to the Army by Fisher. According to the story, Fisher wanted the Army to assign to its new escort fighter a "nice symbolic number", a number that would sound nice in advertising copy and would make for memorable slogans--something like "The French 75 in World War 1, the Fisher P-75 in World War 2" was envisaged. The Army agreed, and skipped the designations P-73 and P-74 and gave Fisher the P-75 designation.
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 a la P-39 Airacobra, and was to drive a set of contrarotating propellers via an extension shaft and a reduction gearbox. The engine was cooled by means of a large 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, and that the tail assembly of the Douglas A-24 (Army version of the SBD Dauntless) and the undercarriage of the Vought F4U Corsair would be used. 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. 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 no less than 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.
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 contrarotating 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.
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.
The P-75A production aircraft featured a modified tail assembly and had a bubble-type canopy replacing the framed and braced cockpit hood of the earlier versions. It featured the V-3420-23 engine of the XP-75A. The first P-75A flew on September 15, 1944. By that time, most of the bugs had been ironed out of the design. However, 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 after only six examples had been built. 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 following specification for the long-range XP-75 are those quoted by the manufacturer; One 2885 hp Allison V-3420-23 twenty-four cylinder liquid-cooled engine. 433 mph at 20,000 feet, initial climb rate 4200 feet per minute st 10,000 feet, 3900 feet per minute at 20,000 feet. Service ceiling was 36,400 feet and absolute ceiling was 29,500 feet. Range with maximum external fuel was 3500 miles. Weights were 11,495 pounds empty, 13,807 pounds normal loaded, and 18,210 pounds maximum. Dimensions were wingspan 49 feet 4 inches, length 40 feet 5 inches, height 15 feet 6 inches, and wing area 347 square feet. Armament consisted of six 0.50-inch machine guns in the wings (235 rpg) located outboard of the propeller arc and four 0.50-in machine guns (300 rpg) in the fuselage nose synchronized to fire through the propeller arc. In addition, a pair of 500-pound bombs could be carried.
The last production P-75A (serial number 44-44553) is on display in the Annex at the Wright-Patterson Air Force Base Museum in Dayton, Ohio. I saw it there in June of 1992. It was in fairly good shape, but needs some restoration work. 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.