The Boeing XP-8 (Model 66) was an experimental design for a fighter aircraft powered by a Packard inverted inline engine.
In April 1925 the USAAC issued a specification for a single-seat fighter to use the 600hp Packard 2A-1500 inverted water cooled engine. Boeing was awarded a bailment contract, in which the Air Corps agreed to provide the engine and military equipment and test the aircraft, but not to actually purchase the aircraft, which remained Boeing's property until January 1928. It was only at this date that the aircraft was officially given the XP-8 designation. This gap explains why the later Model 93, which was purchased early, had the XP-7 designation.
The model 66 used a similar airframe to the Model 15 (PW-9) family. The biggest difference was the radiator. On the PW-9 the radiator was installed in a tunnel below the engine. On the Model 66 it was installed in the centre section of the lower wing which was slightly lowered to make space. The Model 66 was the last Boeing army biplane with tapered wings, and used similar wings to the PW-9, although with the upper wing span reduced and the lower wing span increased.
The Model 66 was delivered in July 1927 and tests began in January 1928. The aircraft was judged a success, but the Packard engine caused too many problems and the aircraft didn't have the performance called for in the 1925 specification. The Model 66 wasn't accepted for production, but some features from its design were used in the Navy's Boeing F2B. The Model 66/ XP-8 was scrapped in June 1929.
Engine: Packard 2A-1500 water cooled inverted V engine
Span: 30ft 1in
Length: 22ft 10in
Height: 10ft 9in
Empty Weight: 2,309lb
Loaded Weight: 3,421lb
Maximum Speed: 173mph at sea level, 166mph at 5,000ft
Cruising Speed: 1,800 ft/min
Climb rate: 2,138ft/ min
Range: 325 miles
Guns: One 0.3in and one 0.5in machine guns