In 1940, the U.S. Army Air Corps issued Request for Proposal R-40C, requesting designs for a high-speed, long-range, high-altitude interceptor intended to destroy enemy bombers. The specifications were very bold, encouraging manufacturers to produce radical aircraft that would outperform any existing fighter in the world at the time. The aerospace parts manufacturer McDonnell Aircraft, eager to begin manufacturing its own aircraft, responded to the proposal with drawings and specifications of the proposed Model I, which would be powered by an unusual geared drivetrain with a single Allison V-3420 engine buried in the fuselage powering twin wing-mounted pusher propellers in the wings.
However, 22 other manufacturers also issued proposals to meet the Army’s request; the McDonnell proposal had relatively unimpressive anticipated performance, and its odd drivetrain was unproven. The Model I fell in 21st place when the 23 proposals were examined and scored. The proposals that were accepted included the similarly ill-fatedXP-54, XP-55, and XP-56. Despite the apparent setback, Air Corps leaders were impressed by the nascent company’s efforts, and granted McDonnell a $3,000 contract to re-engineer the aircraft.
The XP-67’s radical design would require extensive wind tunnel testing to fine-tune its numerous advanced aspects. An extensive aerodynamic test program was begun by three different entities: McDonnell, NACA and the University of Detroit. The design demanded skin that was perfectly smooth and precisely shaped to maintain its laminar-flow characteristics, mandating the development of new construction techniques, as the company had never produced an entire aircraft before. Wind tunnel testing uncovered problems with engine cooling airflow through the engine nacelles, problems that would never be fully resolved. Difficulties were also encountered in obtaining engines, as wartime production demands hampered Continental’s efforts to deliver running examples of the experimental XIV-1430 engines to competing aircraft test programs. The project was also delayed by intense competition for testing time at the NACA wind tunnel facility in Langley, Virginia.
The first XP-67, 42-11677, was ready for ground trials on 1 December 1943, although it was not yet ready for flight. The aircraft was fitted with XIV-1430-17/19 engines and General Electric D-23 turbo-superchargers. No pressurization equipment or armament would ever be installed in the prototype. On 8 December, the aircraft was damaged by fires in both engine nacelles, caused by a malfunction of the exhaust manifold slip rings. By 6 January 1944, the damage was repaired and the XP-67 made its first flight. The flight, however, ended after six minutes due to difficulties with the experimental engines. After a number of modifications were made to the engine installations, two test flights were carried out. On the fourth flight, the engine bearings burned out when the engines were unintentionally overspeeded.
On 23 March 1944, flight trials restarted. U.S. Army Air Forces pilots finally got to fly the aircraft on 11 May 1944, and judged the cockpit layout fair and ground handling satisfactory, but deemed the aircraft underpowered due to its poor initial rate of climb, slow acceleration, and long takeoff roll, particularly when operating with only one engine. Other flight characteristics were generally good during gentle maneuvers; stick forces were light, roll rate was adequate, and control was effective at all speeds with good longitudinal stability.
However, a tendency to dutch roll was prevalent. The prototype also displayed several disturbing behaviors as its stall speed was approached. It began to buffet well above the actual stall speed, it felt tail-heavy in fast turns, and its nose would tuck upwards during the stall. The problems were serious enough that test pilots declined to test the XP-67’s spin characteristics, fearing that a spin might be unrecoverable. This irregular and unstable stall behavior has been attributed to advanced aerodynamic principles that were not fully counteracted until the advent of electronic stability controls years later.Although the final flight test report was generally positive, the aircraft’s maneuverability was deemed inferior to existing types such as the North American P-51 Mustang.
Upon return to the factory, the cooling ducts were reworked. Several problems were cured during the ensuing test flights, but the engines continued to be plagued by chronic overheating and deficient power output. The XP-67 only reached a confirmed top speed of 405 mph, which was far short of its promised top speed of 472 mph, and was unremarkable compared to other fighters in service at the time.
On 6 September 1944, the starboard engine of the XP-67 caught fire during a test flight, and test pilot E.E. Elliot executed an emergency landing at Lambert Field in St. Louis, Missouri. He attempted to park the craft pointing into the wind to blow the flames away from the airframe, but the starboard main landing gear brakes failed, pivoting the XP-67 so the flames blew directly towards the aft fuselage. Elliot escaped safely, but the blaze gutted the fuselage, engine, nacelle and starboard wing; the aircraft was a total loss