The Boeing XP-8 (P-507)was a new design initiated by Boeing to conform to an Air Corps specification issued in April 1925 for a fighter to be powered by the new 600 hp Packard 2A-1500 inverted liquid cooled engine. It was built under a bailment contract, in which the Army agreed to supply the engine and the military equipment and to test the airplane, but Boeing would retain actual ownership of the airframe.
The project was assigned the company designation of Model 66. Although the Model 66 owed a great deal to the PW-9/FB series, it was essentially a new design. The Model 66 was a single-bay biplane with staggered wings that were tapered in chord and joined by a single bay of duralumin N-struts. The span of the upper wing was reduced by almost two feet from the PW-9, while that of the upper wing was increased by the same amount. The fuselage consisted of a welded steel tube framework forward section from the nose to the cockpit, whereas the real of the fuselage including the rudder consisted of a bolted duralumin framework. The wing was made of wood, and both the wings and the fuselage were fabric covered. The radiator was located in the lower fuselage, situated at the point where the bottom wings were joined onto the fuselage. The twin-bladed propeller had an aerodynamically-faired spinner. The forward landing gear struts incorporated oleo-pneumatic shock absorbers, a feature which became standard on subsequent Boeing fighters.
Boeing delivered the Model 66 aircraft to the Army in July 1927. It flew for the first time in July 14, 1927. The designation XP-8 was not assigned to the aircraft until the USAAC bought the plane from Boeing on a separate contract signed in January 1928. The Army assigned it the serial number 28-359.
Testing showed that the performance of the XP-8 was rather disappointing, and the Army decided not to order the aircraft into production. The Achilles heel of the XP-8 was in its Packard engine, which was still in the experimental stage. Performance included a maximum speed of 173 mph, an initial climb rate of 2138 ft/min, a service ceiling of 23,000 feet, and a range of 325 miles. Weights were 2390 lbs. empty and 3421 lbs. gross. Armament consisted of one 0.30 cal and one 0.50 cal machine gun in the upper fuselage decking synchronized to fire thru the propeller
The sole XP-8 was surveyed at Wright Field in June 1929. The plane appears not to have been scrapped, but was instead transferred to the Department of Commerce. The Department eventually sold it to a private owner, but the plane never appeared on the civil register.