In June 1925, before the first of the Boeing PW-9s ordered in 1924 had been delivered, the USAAS decided to have the 30th and last PW-9 aircraft (Ser No 25-324) modified during production to test the high-altitude performance of the new 510 hp Packard 1A-1500 turbosupercharged engine. This modification was considered important enough to warrant a designation change to XP-4 in accordance with the new system which had just been adopted.
The XP-4 had a new, more aerodynamic wing profile, with both wings being of equal size and larger than those of the PW-9 in order to provide the necessary larger area needed to lift the increased weight. The fuselage-mounted pair of 0.30-cal machine guns were supplemented by two extra 0.30-cal machine guns mounted in the lower wing, situated far enough outboard to clear the propeller arc. A four-bladed propeller was fitted. The new engine made it necessary to redesign the cowling, and the turbosupercharger was mounted externally on the right-hand side.
The XP-4 was delivered to the Army at Wright Field for tests on July 27, 1926. The Packard engine did not prove to be sufficiently powerful to compensate for the 800-lb increase in empty weight, and the performance of the XP-4 was rather disappointing. It did, in fact, actually perform more poorly than did the standard PW-9, and the XP-4 project was abandoned after only four hours of flying time. The airframe was surveyed on May 1, 1928.
Empty weight was 2711 lbs, with gross weight being 3603 lbs. Maximum speed was 161 mph. Initial climb rate was 2055 ft/min. Service ceiling was 22,000 feet, and range was 375 miles.