The last aircraft built by Boeing against an order for 30 PW-9 single-seat fighters was retained by the company at the request of the USAAC for trials with a turbo-supercharged version of the 510hp Packard 1A-1500 water-cooled engine (utilised by the FB-2). The USAAC also requested an increase in wing area, because of the higher weights involved, and while the standard PW-9 upper wing was retained, an enlarged lower wing was introduced, resulting in an equi-span biplane with vertical N-type interplane struts. The standard PW-9 armament of two 7.62mm synchronised machine guns was augmented by a pair of similar-calibre weapons mounted beneath the lower wing outside the propeller arc. With these changes the aircraft was designated XP-4 (Model 58), and delivered on 27 July 1926, but the 6.32m² increase in gross wing area by comparison with the PW-9 failed to compensate for the 370kg increase in loaded weight. The XP-4 proved to possess extremely unsatisfactory handling characteristics and after a few hours flying the test programme was abandoned.