On March 11, 1939, the USAAC Materiel Division issued Circular Proposal 39-775 to the aircraft industry. This proposal called for a new type of twin-engined, high-performance interceptor fighter. The successful entry was, however, to derive as many design features as possible from already existing aircraft.
Four contractors submitted proposals. The Lockheed entry was a progressive development of the P-38 Lightning, and was given the company designation of Model 222. The Model 222 had the same general arrangement as the P-38, but featured a pressure cabin and was powered by a pair of turbosupercharged twenty-four cylinder Pratt & Whitney X-1800-SA2-G (military designation XH-2600) liquid-cooled engines which were supposed to develop somewhere between 2000 and 2200 horsepower. Lockheed proposed to replace these engines by a pair of 2300 hp Wright R-2160 Tornado turbosupercharged radials in production aircraft. Armament was to be a pair of 20-mm cannon and four 0.50-inch machine guns. Total fuel capacity was to be 300 US gallons, as compared to 230 US gallons for the early production P-38. The Model 222 was rather optimistically estimated to have a top speed of 473 mph at 20,000 feet when powered by the Pratt & Whitney XH-2600s, and a speed of no less than 500 mph at the same altitude when powered by the Wright Tornadoes.
The USAAC finished looking over the four proposals on August 3, 1939. The Lockheed proposal (which by this time had had its company designation changed to Model 522) was judged the most promising of the four entries, and the USAAC ordered one example under the designation XP-49 in October 1939. The competing Grumman entry was their Design 41, which was a development of the XF5F-1 Skyrocket twin-engined carrier-based fighter. The Grumman design came in second, but the USAAC considered it sufficiently promising that they ordered one example under the designation XP-50.
A contract for a single XP-49 prototype was officially issued on January 8, 1940. Because the Lockheed company was preoccupied with the P-38 Lightning, work on the XP-49 proceeded quite slowly during the early months of 1940. Both the USAAC and Lockheed soon came to realize that with either the Pratt & Whitney XH-2600 or the Wright R-2160 engines, the XP-49 would be seriously overpowered. Consequently, in March 1940 it was decided to substitute a pair of experimental Continental XIV-1430-9/11 twelve-cylinder inverted-vee liquid-cooled engines rated at 1540 hp for takeoff. In order to counteract torque, the engines rotated in opposite directions--the port propeller rotated CCW when viewed from the rear, and the starboard propeller rotated CW. Other changes included the substitution of dummy armor plate for the genuine armor plate called for in the original specification, thus expediting construction of the prototype. A maximum speed of 458 mph at 25,000 feet was now anticipated.
On December 23, 1940 , detailed design of the XP-49 began under the direction of project engineer M. Carl Haddon. Two-thirds of the XP-49 airframe components were common with the P-38. The primary differences were in the engine installation, the use of a heavier and stronger undercarriage, and a pressurized cockpit similar to that of the XP-38A.
Since much of the airframe was common with the production P-38, the construction of the XP-49 prototype (serial number 40-3055) went fairly rapidly. However, the first flight was delayed by problems with the experimental Continental engines, which were not yet cleared for flight operations at the time they were delivered to Lockheed in April 1942. It was not until November 14, 1942 that the XP-49 took to the air for the first time, flown by test pilot Joe Towle.
The aircraft was grounded only a week later for replacement of the engines by XIV-1430-13/15 engines rated at 1350 hp for takeoff and 1600 hp at 25,000 feet. The fuel tanks were replaced by self-sealing tanks taken from a P-38, and a flight engineer's jump seat was added behind the pilot's seat. Flights were resumed in December, but were marred by continual hydraulic problems. When it was actually able to fly at all, the aircraft handled fairly well and had good maneuverability, but the Continental engines gave the XP-49 a rather uninspiring performance--the maximum speed was only 406 mph at 15,000 feet as against a promised speed of 458 mph at 25,000 feet.
On January 1, 1943, the XP-49 was damaged during an emergency landing at Muroc AAB after a simultaneous inflight failure of both the hydraulic and the electrical systems. While being repaired, the XP-49 received 7 3/4 inch taller vertical tail surfaces. The XP-49 flew again on February 16, 1943. In this form, it was delivered to Wright Field on June 26, 1943, almost 27 months later than expected. By that time, the Army had lost all interest in the XP-49, since the performance was actually inferior to that of the standard P-38J which was already in service. In addition, the questionable future of the troublesome Continental engine caused the Army to abandon any further consideration of quantity production of the XP-49.
Even after the USAAF had decided not to proceed with quantity production of the XP-49, the Army continued testing the aircraft at Wright Field. However, maintenance difficulties with the Continental engines and problems with the fuel system limited the usefulness of the XP-49, and it was flown only rarely. It ended its useful life by being dropped from a bridge crane to simulate hard landings. It was finally scrapped in 1946.
Performance of the XP-49 included a maximum speed of 406 mph at 15,000 feet, 384 mph at 10,000 feet, and 347 mph at sea level. Initial climb rate was 3300 feet per minute, and the XP-49 could climb to 20,000 feet in 8.7 minutes. Normal range was 679 miles, and maximum range was 1800 miles. Service ceiling was 37,500 feet. Weights were 15,410 pounds empty and 18,750 pounds loaded. Wingspan was 52 feet 0 inches, length was 40 feet 1 inch, height was 9 feet 9 1/2 inches (original tail), 10 feet 5 1/4 inches (revised tail), and wing area was 327.5 square feet. The proposed armament of 2 20-mm cannon with 670 rpg and four 0.50-inch machine guns with 300 rpg was never actually fitted.