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The Boeing XP-4 (Model 58) was an experimental aircraft produced to test a turbo-supercharged Packard engine. It was produced by modifying the last of the thirty Boeing PW-9. The PW-9 was an unequal span biplane, with a welded steel tube fuselage, wooden wings, both with a fabric covering. It had tapered wings, with the lower wing significantly smaller than the upper.
The last PW-9 was completed as the only XP-4 (Model 58). It was powered by a 510hp turbo-supercharged Packard 1A-1500 water-cooled engine. The small lower wing of the PW-9 was replaced with a wing with the same dimensions as the upper wing. Both wings had a new Boeing designed aerofoil section and carried ailerons. The wings were linked by N struts. The XP-4 carried the normal two nose-mounted machine guns but was also given two extra machine guns carried in lower wings, outside the propeller disc.
The XP-4 was an unsuccessful design. It was delivered on 27 July 1928, and the test programme began. It quickly became clear that the larger wings were still not large enough to cope with the extra weight of the aircraft, and the tests were abandoned after only four flying hours. The aircraft survived until 1 May 1928 when it was written off the Army's active list.
Engine: Turbo-supercharged Packard 1A-1500 water-cooled engine
Power: 510hp
Crew: 1
Span: 32ft 0in
Length: 23ft 11in
Height: 8ft 10in
Empty Weight: 2,783lb
Loaded Weight: 3,650lb
Maximum Speed: 161mph
Cruising Speed: 137mph
Climb rate: 2,055ft/ min
Ceiling: 22,000ft
Range: 375 miles
Guns: Four 0.3in machine guns