The McDonnell XP-67, otherwise known as the “Bat” or “Moonbat”, was a ambitious prototype for a twin-engine, long range, single-seat interceptor for the USAAF. The XP-67 prototype was the product of a 1940 US Army Air Corps request for a high-altitude, long range aircraft designed to take out enemy bombers. At the time McDonnell were a fairly new aerospace parts manufacturer keen to develop their own aircraft. In all 23 manufacturers submitted proposals to the Air Corps, and out of those 23 McDonnell’s design came a disappointing 21st. However, despite their low ranking, the military had been impressed by the company’s efforts to produce a radical and innovative design and they awarded them a $3,000 contract to re-engineer the aircraft.
McDonnell went away and subsequently redesigned the aircraft two more times before the USAAC gave the aircraft its approval for further development. In 1941 the company secured a $1,508,596 contract, plus an $86,315 fee, for two prototypes, a wind tunnel model, and related engineering data. The XP-67 was born.
The McDonnell XP-67 was a futuristic prototype which featured a smooth laminar-flow airfoil section throughout the design, a pressurized cockpit – unusual for the time, and it was to be heavily armed with a combination of machine guns and cannons. The complicated aerodynamic shape of the aircraft was developed through a test program conducted by McDonnell, NACA and the University of Detroit. Wind tunnel testing showed problems with cooling airflow for the engines, a problem which was never rectified and ultimately doomed the project.
The McDonnell XP-67 began ground tests in December 1943. It was fitted with experimental Continental XIV-1430-17/19 engines and General Electric D-23 turbo-superchargers. Just a week later the aircraft was damaged by fires in both engines, caused by a malfunction in the manifold slip rings. By January 6th 1944 the aircraft had been repaired and it went for its maiden flight. However just 6 minutes later it was back on the ground due to engine problems. After further modifications to the engines it took to the skies again, but on the fourth test flight the engine bearings burnt out.
While the engines were proving to be unreliable, it was also becoming apparent that they were also underpowered. Continental had promised the engines would each deliver 1,350 hp, but as it turned out they were only providing 1,060 hp – effectively robbing the XP-67 of 580 much-needed horsepower.
At this point, Jim McDonnell, the company’s founder, began a campaign to raise funding from the military to redesign the aircraft and refit it with Allison or Rolls Royce piston engines, backed up by Westinghouse turbojets in the rear nacelles. The Army turned down his request, instead insisting on more tests of the original design.
On March 23rd 1944 flight trials resumed, and in May of that year Army test pilots finally got their hands on the aircraft. The XP-67 was judged to offer satisfactory ground handling and a comfortable cockpit. However it was also deemed to be underpowered – something McDonnell already knew and with time would have surely rectified. Perhaps more alarmingly was the fact the XP-67 had some unusual flight characteristics, especially at low speed. It began to buffet well above its actual stall speed, in addition its nose would tuck upwards during the stall. In high-speed turns pilots reported that the tail felt unusually heavy. The unusual flight dynamics were alarming enough that no test pilot agreed to trial the aircraft’s spin characteristics, fearing that any spin would be unrecoverable. It has later been found that the flight stability problems with the XP-67 were probably due to the sophisticated laminar flow design of the aircraft, and had it been a developed now, and fitted with electronic stability aids like all modern aircraft, these problems would have been easily rectified.
On September 6th 1944 the XP-67 was being piloted by E.E. Elliot during a test flight at Lambert Field in St. Louis Missouri when the starboard engine caught fire. Elliot managed to bring the aircraft down for an emergency landing and he attempted to park the aircraft so the wind blew the flames away from the fuselage. However on landing the starboard brakes failed causing the aircraft to spin around and point directly into the wind. Elliot managed to escape the aircraft safely, but the fire rapidly consumed most of the aircraft. It was unsalvageable.
At the time the second prototype was only 15 per cent complete. After the first prototype’s demise, the Army quickly moved to cancel the program and on 13th September 1944 it was all over for the XP-67. What was left of the first prototype was scrapped, and work on the second halted.
In the end the McDonnell XP-67 was perhaps the victim of its overly ambitious design, but it was also badly let down by its underperforming and unreliable engines. Had these lived up to their initial promises then perhaps it would have been a different story.