History
Background
On November 27, 1939, the US Army Air Corps issued a Request for Proposal R-40C that sought a high-speed, long-range, high-altitude interceptor with improved performance, armament, and pilot visibility over existing fighters. The interceptor was to be specialized to exterminate enemy bombers. The specifications already outlined that the interceptor must be capable of reaching 20,000 feet in 7 minutes and attaining 425 mph at 15,000 to 20,000 feet. The armament would be either at least four guns or cannons. The specifications were very ambitious, and they intended to encourage manufacturers to produce innovative interceptors that would outperform any existing fighter in the era. In addition, they allowed the manufacturers to come up with unconventional, radical aircraft designs.
Just 23 other manufacturers had issued proposals to meet the Army’s request; the XP-54, XP-55, and XP-56 designs were among the accepted proposals.
Design and Development
McDonnell Aircraft Corporation of St Louis, Missouri, was incorporated on July 6, 1939, and had submitted a proposal for Model I design in response to the US Army Air Corps’ R-40C. It failed to win the entry to the R-40C competition in 1940. It only placed 21st place when the 23 proposals were examined and scored. Nevertheless, the Model I design intrigued the US Army Air Corps enough to purchase the engineering data.
The Model I initially would be powered by a geared drivetrain with a single Allison V-3420 engine buried in the fuselage, powering twin wing-mounted pusher propellers in the wings. McDonnell engineers returned on June 30, 1940, with the Model II, which also got rejected. The engineers continued to revise the design with feedback from Material Division engineers. This turned into an iterated design known as the Model II-A on April 24, 1941. This new design was powered by two 1,350-hp XIV-1430-1s with General Electric D-2 turbo-superchargers. The engine exhaust gases were ejected from central nozzles at the end of each nacelle to augment thrust. Its general arrangement was for a conventional single-seat twin-engined monoplane with a tricycle undercarriage. The laminar airfoil sections were used throughout, and the nacelles were buried in the wings to reduce the air drag. The aircraft would have a pressurized cockpit with a sliding canopy for one pilot. Its initial armament was planned to include six .50-cal. machine guns and four 20-mm weapons. This was later changed to six 37-mm cannons with 45 rpg in the inboard wing sections. McDonnell accepted a guarantee for 372 mph at 18,600 pounds, and the maximum climb rate was expected to be 3,200 feet per minute, although the anticipated gross weight was increased to 20,000 pounds.
The Army was satisfied with the new design proposal and awarded McDonnell a contract on October 29, 1941, to initiate the development of two Model II-A prototypes, officially designating them XP-67. McDonnell would build a wind tunnel model and submit associated engineering data as ordered in the contract.
Flight Testing
The contract schedule had called for the completion of the prototype by the end of April 1943. However, the difficulties discovered during the wind tunnel testing delayed its development. The issue with engine cooling airflow through the engine nacelles was identified. In October 1943, the Army suspended the construction of the second prototype, as they were seeking solutions but found none. There was another difficulty in obtaining engines, as wartime production demands hampered the efforts to power the prototype, delaying the schedule even longer.
The first XP-67 (42-11677) was finally ready for ground trials on December 1, 1943, fitted with XIV-1430-17/19 engines and turbo-superchargers but no pressurization equipment and armament. It began taxiing on the ground, and the engines immediately caught on fire in both nacelles during a run-up on December 8, 1943, at Lambert Field in St Louis. The fires were caused by a malfunction of the exhaust manifold slip rings.
The XP-67 was changed and repaired before it was ready for flight testing on January 6, 1944, at Scott Field in Illinois. On that day, the XP-67 lasted six minutes of flight before its engines experienced problems. The aircraft was grounded while modifications were made to the engine installations in the nacelles, and then it conducted two test flights. On the fourth flight, February 1, 1944, the engine bearings burned out when the engines were accidentally overspeeded.
The damage made to the engines was so severe that the aircraft had to return to McDonnell in St. Louis for further modifications and replacement engines. Wind tunnel testing had recommended that the tailplane be raised one foot to improve longitudinal stability, and this modification was carried out on the aircraft while it was waiting for the replacement engines.
The flight testing resumed on March 23, 1944. This time, the Army pilots finally got to fly the aircraft on May 11, 1944, and made five more successful flights during the month. The pilots were underwhelmed with the aircraft. They reported it was felt to be underpowered, with unimpressive acceleration and poor climb performance. There were stability issues with the aircraft in that it tended to dutch roll, and it was reported to be generally less maneuverable than the existing Army fighters. However, the pilots reported that the cockpit layout was adequate and that ground handling and handling in the air were satisfactory. The roll rate was also acceptable at high speed.
The issues with weak flight performance were attributed to the experimental engines. The engines gave 300 horsepower less output than the promised 1,350 horsepower. In addition, the gross weight was increased over the guaranteed maximum, totaling 25,400 pounds. Nevertheless, the overweight aircraft eventually demonstrated 405 mph at 25,000 feet and 22,000 pounds weight but still below guarantee. The XP-67 was returned to McDonnell to have its cooling ducts reworked, and several problems during the flights were ironed out. However, they could not fix the deficient power output from the engines.
Test flights for the XP-67 continued throughout the summer of 1944, with a dorsal fin and two degrees of dihedral added to the tailplane to improve lateral stability. The aircraft was planned to conduct the official performance tests in September 1944 at Lambert Field in St. Louis, Missouri. However, on September 6, 1944, the aircraft caught on fire in the right engine nacelle while taking off for the test flight. It safely landed and attempted to park into the wind to blow the flames away from the airframe, but the starboard landing gear brakes failed. It pivoted the aircraft, so the wind blew the flames over the fuselage and caused severe damage to the aircraft beyond repair.
Epilogue
The XP-67 (42-11677) was written off as a total loss. This aircraft had logged 43 flight hours before being out of action. The tragic accident, along with the engine problems, caused the XP-67 project to be halted. Subsequently, on October 24, 1944, both McDonnell and the Army decided to terminate the project for good. Although the final flight test report was generally positive, the aircraft offered no significant advantages over existing fighters already in service.
Another XP-67 (42-11678) was only 15 percent complete at the time and halted upon the cancellation. This prototype was to have been powered by I-1430 liquid-cooled engines with war emergency power ratings increased to 2,100 hp. Contra-rotating propellers were also to be fitted in place of the propellers of the prototype. It was planned to have an unusual four-engine configuration consisting of two Packard or Allison piston engines and two turbojet engines. Its armament would have four 20-mm cannons with two 37-mm cannons. However, this prototype was never completed and was abandoned since the jet fighters of superior performance were already becoming available, with no plans made for continuing further with the XP-67 project. All prototypes were scrapped.
Although McDonnell failed to deliver its first fighter aircraft, it later became successful with the FD-1 Phantom development, which was the US Navy’s first jet fighter and would earn the corporation a reputation as a giant in the aviation industry.
