The McDonnell Aircraft Corporation of St Louis, Missouri was incorporated on July 6, 1939. McDonnell was certainly not a company to start small--as its first military project, the company immediately began a design study for a long range fighter. A rather ambitious first project for such a new aircraft corporation!
In June of 1940, the new McDonnell company submitted an unsolicited proposal to the USAAC for an unconventional fighter powered by either an Allison V-3420-B2 or a Pratt & Whitney H-3130 engine equipped with a two-stage supercharger. The engine was to be buried in the fuselage aft of the pilot. The engine was to drive a pair of pusher propellers situated aft of the wings by means of a complicated system of extension shafts and gear drives. Although the USAAC rejected the proposal because of excessive weight and complexity, the Army was sufficiently impressed that it bought the engineering information from McDonnell on June 6, 1940 and encouraged the new company to keep trying.
The next McDonnell proposal to the Army came later that same month. This time the company issued a proposal for a twin-engined, two-seat heavy fighter powered by a pair of Continental I-1430 twelve-cylinder inverted-Vee liquid-cooled engines. Initially, the USAAC expressed no interest, but during subsequent discussions with McDonnell some revisions were made and on May 5, 1941 a formal proposal was submitted to the Army. The project was given the company designation of Model S-23-A and called for a single-seat long-range fighter with a pressurized cabin. An unusually heavy armament of six 0.50-inch machine guns and four 20-mm cannon was proposed. McDonnell's design team attempted to maintain true aerofoil sections throughout the entire airframe, the center fuselage and the rear portions of the engine nacelles merging smoothly together. This gave the aircraft a unique bat-like appearance. The forward member of the tricycle undercarriage retracted into the fuselage, whereas the mainwheels retracted into the engine nacelles. Guaranteed maximum speed was a rather optimistic 472 mph at 25,000 feet, and gross weight was estimated to be 18,600 pounds.
This time the Army was definitely interested, and on August 2, 1941 the USAAF issued an authority for purchase of two prototypes. The contract was formally approved on October 29, 1941. The designation XP-67 was assigned, and the serial numbers of the two prototypes were 42-11677 and 42-11678.
By the time that detailed design of the XP-67 got underway, the armament was changed to six 37-mm cannon with 45rpg in the inboard wing sections. The pair of Continental XI-1430-1 engines were fitted with General Electric D-1 turbosuperchargers and drove four-blade propellers. A unique idea was to use the engine exhaust to augment the thrust. The increased armament and other changes caused the estimated gross weight to rise to 20,000 pounds.
With the exception of armament, cabin pressurization equipment, and the oxygen system, the first XP-67 (42-11677) was ready by December 1, 1943. The aircraft had conventional ailerons rather than the planned drooping ailerons. By this time, the engines were XI-1430-17/19s, with D-23 turbosuperchargers. The Continental engines were rated at 1350 hp for takeoff and 1600 hp at 25,000 feet.
The initial flight tests of the XP-67 were delayed by fires in BOTH engines that broke out during a high-speed taxiing run at Lambert Field in St Louis on December 8, 1943. After being repaired, the XP-67 was trucked to Scott Field in Illinois. The first flight of the XP-67 took place there on January 6, 1944 with test pilot E. E. Elliott at the controls. However, this flight had to be abruptly terminated after only six minutes owing to engine problems.
The XP-67 was grounded while modifications were made to the engine compartments. A stainless-steel bulkhead was installed to seal off the turbosupercharger compartment from the rest of the engine, improvements were made to the cooling air circulatory system, and the aft ends of the engine cowlings were shortened. With these modifications, two test flights were completed successfully. However, on the fourth flight (on February 1, 1944), the Continental engines were deliberately overspeeded and the bearings burned out, forcing yet another emergency landing.
This mishap had damaged the Continental engines beyond repair, and since replacements were not immediately available, the XP-67 was returned to McDonnell in St Louis for modifications. Wind tunnel testing had suggested that the tailplane should be raised one foot in order to improve longitudinal stability, and this was done while the aircraft was awaiting replacement engines.
Test flying resumed on March 23, 1944, and five more successful flights were made during the month of May by USAAF test pilots. During this time, some problems were encountered with engine roughness, with improper aileron balance, and with unsatisfactory main undercarriage door closure. However, the USAAF pilots reported that the cockpit layout was adequate and that ground handling was satisfactory. Handling in the air was considered as being satisfactory and the roll rate was deemed to be good at high speed. The fighter was stable longitudinally, but it was neutrally stable laterally and tended to "Dutch roll".
However the performance of the XP-67 fell quite short of that which was promised. The takeoff run was excessively long, the initial climb rate was poor, and the acceleration was slow. The aircraft was clearly underpowered with its troublesome Continental engines, which failed to develop their design rating of 1350 hp, barely reaching 1060 hp.
Test flights continued throughout the summer of 1944. A dorsal fin and an additional two degrees of dihedral were added to the tailplane to improve lateral stablity. The XP-67 was scheduled to begin official performance tests in September, but before they could get underway a fire broke out in the right engine nacelle while test pilot E. E. Elliott was taking the XP-67 for a test flight. Elliott safely landed the aircraft, but the wind blew the flames over the fuselage and caused major damage to the structure.
The XP-67 was deemed to have been damaged beyond economical repair. This accident, plus the seemingly endless series of problems caused by the temperamental Continental engines, caused the USAAF to recommend that work on the XP-67 project be halted. On September 13, both McDonnell and the USAAF agreed that the project should be terminated. The contract was formally cancelled six weeks later.
The second prototype (42-11678) was cancelled before it could be completed. This prototype was to have been powered by I-1430 liquid-cooled engines with war emergency power ratings increased to 2100 hp. Contrarotating propellers were to be fitted in place of the handed propellers of the first prototype. There was even some talk of fitting a mixed powerplant arrangement to later production P-67s, with either a Packard V-1650 or an Allison V-1710 engine with two-stage supercharger in front of each engine nacelle and an I-20 turbojet in the rear of each nacelle. However, nothing ever came of these plans, since the USAAF requirement for long-range escort fighters was more than adequately satisfied by the North American P-51H Mustang, the Republic P-47N Thunderbolt, and later by the North American P-82 Twin Mustang.
The maximum speed attained by the XP-67 during tests was 405 mph at 15,000 feet and 357 mph at 10,000 feet. Initial climb rate was 2600 feet per minute, and service ceiling was 37,400 feet. Maximum range was 2385 miles. Weights were 17,745 pounds empty, 23,114 pounds loaded, 15,400 pounds maximum. Dimensions were wingspan 55 feet 0 inches, length 44 feet 9 1/4 inches, height 15 feet 9 inches, and wing area 415 square feet. The planned armament of six 37-mm cannon was never actually fitted.