02/08/2026. Remarks by Johan Visschedijk: "The last Boeing Model 15 PW-9 was converted to the Model 58 XP-4, a new experimental fighter design, by replacing the original 435 hp Curtiss D-12 twelve-cylinder liquid-cooled V-engine for a similar Packard 1A-1500 engine of 510 hp, fitted with a turbo-supercharger .

A new lower wing having the same dimensions as the PW-9 upper wing of 32 ft 0 in (9.75 m) span replaced the regular 22 ft 5 in (6.83 m) lower sesqui-wing. There was no increase in the gap between the wings. The aerofoil section was changed from the original Gottingen 436 to a new Boeing 103A section. The armament was revised by installing a 0.30 in (7.62 mm) caliber gun in each lower wing far enough outboard to clear the propeller.

(David Gauthier Memorial Collection)

The XP-4 was 815 lb (370 kg) heavier than the PW-9 but had only 46 sq.ft (4.27 sq.m) of additional wing area. It was severely handicapped in that it was merely an adaption of an existing aircraft to a new configuration instead of having been designed for the best utilization of the new features. The XP-4 was delivered to Wright Field on July 27, 1926, and was surveyed on May 1, 1928, since it was of no further use as a test aircraft and was too different from standard models for a service or advanced training squadron."

Span: 32 ft 0 in (9.75 m)
Length: 23 ft 11 in (7.29 m)
Height: 8 ft 10 in (2.69 m)
Wing area: 309 sq.ft (28.71 sq.m)
Empty weight: 2,783 lb (1,262 kg)
Loaded weight: 3,650 lb (1,656 kg)
Max speed: 161 mph (259 kmh)
Cruise speed: 137 mph (220 kmh)
Climb: 2,055 ft (626 m)/min
Service ceiling: 22,000 ft (6,858 m)
Range: 375 mls (604 km)