CRISIS-BORN FIGHTER PROJECTS, XP-48 TO XP-77
Now that the single-seat types that did most Air Force fighter work in World War II have been described, attention can be given the numerous experimental projects begun by the Materiel Division. In the two years between the outbreak of war in Europe and the attack on Pearl Harbor, an extraordinary variety of designs received pursuit designations from XP-46 to XP-71. Nevertheless, only three of over two dozen projects reached production in time to join the P-38, P-39, and P-40 types already on order when Germany’s invasion of Poland began the war.
The first flurry of designs were the XP-46 to XP-50 fighters offered in the August 1939 design competitions. None passed the prototype stage, since the Curtiss XP-46, Republic XP-47, and Lockheed XP-49 were submerged by P-40, P-47B, and P-38 developments. Douglas made its only attempt at a land-based fighter with the XP-48 (Model 312), but it was never built.
That Douglas specification, dated August 5, 1939, was for a lightweight 350-mph single-seater with a 525-hp Ranger SGV-770, tricycle gear, and two guns. Weight was expected to be only 3,400 pounds, with a wide, narrow wing requiring only 92 square feet of area. By February 1940, the Air Corps had rejected this approach.
Instead, prototypes were ordered of the top two competitors, the XP-49 and XP-50, in the August 1939 competition for twin-engine fighters. Lockheed attempted to improve the P-38 by using more advanced engines. This project, known as the XP-49 (Lockheed Model 522), began with a company proposal to the Air Corps in August 1939. A prototype contract was ready by November 30, but was not finally approved until January 22, 1940. Delivery in 14 months was promised.
The first specification called for the proposed Pratt & Whitney X-l800 (H-2600) 24-cylinder engine, and a speed of 473 mph at 20,000 feet was anticipated. When this engine was canceled, the Continental XI-1430, an inverted inline engine with 27% less frontal area than the Allison, was substituted by September 9, 1940. The new specification guaranteed top speed would be 372 mph at 5,000 feet, and 458 mph at 25,000 feet, where 1,600-hp per engine was expected.
Flight tests began on November 14, 1942, 33 months after the original contract, and continued until the following June with discouraging results. Top speed was only 361 mph at 5,000 feet and 406 mph at 15,000 feet. Armament was to include two 20-mm guns with 60 rpg and four .50-caliber guns with 300 rpg, but neither weapons or a proposed pressurized cabin were ever installed. The XP-49’s only value was as a test bed for the Continental engine.
Grumman’s only Army fighter, the XP-50, was a twin-engine counterpart of the Navy’s XF5F-l, with a short squared-off wing, stubby fuselage, twin rudders, and tricycle landing gear. Equipment included front and back pilot armor, two 20-mm and two .50-caliber guns in the nose, and a rack for two 100-pound bombs. The XP-50 was ordered November 29, 1939, and since the power plant choice, turbosupercharged Wright Cyclones, was less speculative than that of the rival XP-49, the Grumman was first in the air. A maiden flight was made February 18, 1941, but a turbosupercharger explosion on May 14 caused the loss of the $353,828 prototype.
Undaunted, Grumman then proposed its Model 51 with two 1,700 hp Wright R-2600-10 Double Cyclones, and two prototypes designated XP-65 were recommended May 19, 1941, and authorized June 16. Essentially this design was like the Navy’s XF7F-l prepared at the same time, but the Army wanted turbo supercharging, a pressurized cabin, and two 37-mm guns. The Army came to feel that “commonalties” with Navy design were a handicap, and since Grumman’s concentration was on Navy projects, cancellation was recommended December 15, and completed by January 16, 1942.