The final production PW-9D (the 16th) was retained by Boeing for the experimental installation of a 600hp water-cooled Curtiss Conqueror V-1570 engine and, as the XP-7 (Model 93), was eventually delivered to the USAAC on 4 September 1928. Distinguished from the PW-9D by a shorter, deeper nose with a larger radiator, the XP-7 enjoyed some success during trials, proving the suitability of the Conqueror for fighter installation, but proposals to build four service evaluation P-7s were As the XP-7, the last production PW-9 was used to test the Conqueror engine for fighter use. abandoned when it was concluded that the basic PW-9 airframe had reached the limit of its development. The XP-7, the last Boeing fighter biplane to employ a liquid-cooled engine, was reconverted to PW-9D standards.