The Boeing PW-9 is not in the P-for-pursuit sequence, but some details about it are needed for what is to come later.

In 1922, William Boeing committed his company to a private venture which would, it was hoped, produce a fighter that would compare favorably with contemporary British and French designs. Heretofore, Boeing's only experience with fighters had been in the production of the Thomas-Morse-designed MB-3A.

    The project was given the company designation of Model 15. The Model 15 owed a great deal to an examination of the Fokker D.VII, many examples of which had been brought to the USA after the end of the First World War under the terms of the Armistice. The fuselage was of welded steel tubing construction put together by arc-welding (a Boeing-developed technique). The aircraft was a single-bay biplane with tapered wings linked together by an N-strut. The lower wing was shorter and narrower than the upper wing. The wings were of traditional wooden frame construction with fabric covering. The aircraft was originally to have been powered by the same 300 hp Wright-Hispano engine that had powered the Boeing-built MB-3A. However, early in the design process, it was decided to switch to the 435hp 12-cylinder liquid-cooled Curtiss D-12 (later known as V-1150) engine. A tunnel radiator was mounted underneath the nose for engine cooling.

    Although the Army was definitely interested in the Boeing Model 15, they were not able to support the project financially. Money for any sort of new military hardware was hard to come by in the early 'twenties. However, on Apr 4, 1923, the Army signed a bailment contract for the Model 15, in which they agreed to provide the armament and the power plant and also agreed to test the airplane. Boeing would retain the airframe and all proprietary design rights.

Model 15 made its first flight on Apr 29, 1923. It was sent to McCook Field in June 1923 for testing in the Army's 1923 Pursuit Competition. While there, the Boeing design came into direct competition with the basically similar Curtiss PW-8. The Curtiss company had been so miffed by the sudden appearance of the upstart competitor from Boeing that they had initially attempted to delay delivery of a D-12 engine to the Army for installation in the Model 15.

    Both the Boeing and the Curtiss aircraft quickly demonstrated that they were far superior to the MB-3A which had just entered squadron service. Although the Curtiss design was slightly faster, the Boeing design was more maneuverable, more rugged and easier to maintain. In spite of a preference for the Boeing design, on Sep 25, 1923, the Army placed an order for 25 Curtiss PW-8s, in exchange for the Curtiss company's agreement to participate in General Billy Mitchell's scheme for a dawn-to-dusk transcontinental crossing of the USA. Certain features of the Boeing design such as the tapered wing and the tunnel radiator so impressed the Army that they asked Curtiss to try them out on the XPW-8A. This resulted in the XPW-8B, which became the prototype for the famed Curtiss Hawk series of pursuits.

    The Boeing company's financial gamble finally paid off on Sept 28, 1923. On that day, the USAAS purchased the Model 15 under the designation XPW-9 as [23-1216] and ordered two more prototypes [23-1217/1218]. The contract specified that certain changes needed to be incorporated in the second and third prototypes to make them conform to Air Service specifications. The second and third prototypes were delivered to the Army on May 1, 1924. A new Boeing-designed undercarriage with divided axle was later tested on XPW-9 number 1, after it had initially been delivered with the Fokker-like straight axle.

The USAAS ordered an initial batch of 12 production PW-9s on Sept 19, 1924 [25-295/306] and a second batch of 18 on Dec 16, 1924 [25307/324]. The first delivery took place on Oct 30, 1926. The production PW-9 was based on the third XPW-9 prototype, but differed from it in certain detail—engine cowling was better streamlined, the divided-axle undercarriage with 28x4 wheels was adopted as standard, and the center section fuel tank was eliminated. The wheels were changed in service to the larger 750x125mm size.

    Production PW-9s were shipped to US bases in Hawaii and the Philippines. One PW-9 was fitted at McCook Field with all-metal wings for tests.

    In June 1925, before the first of the PW-9s had been delivered, the USAAS decided to have the 30th and last PW-9 [25-324] modified during production to test the high-altitude performance of the turbo-supercharged 510hp Packard 1A-1500. The modification was considered important enough to warrant a designation change to XP-4 in accordance with the new system which had been adopted in May of 1924.

    Even though the production version of the third Curtiss XPW-8, which had been modified as XPW-8B with tapered wings and a tunnel radiator, was built as P-1 under the new P-for-pursuit designation system, the old PW-designation was retained by the Boeing design all throughout its production run.

The PW-9A was basically similar to the PW-9 but was powered by the later Curtiss D-12C engine (later redesignated V-1150-1). 25 examples were ordered on Oct 26, 1925. [26-351/375]. The only noticeable outward change was that the landing wires were doubled. First delivery took place on June 19, 1926.

    PW-9A [26-374] was converted to AT-3 (Boeing Model 68), an experimental single-seat advanced trainer powered by a 180hp war-surplus Wright-Hispano E. A similar sort of conversion was made by Curtiss when the same type of engine was installed in a Curtiss P-1A airframe, creating the AT-4.

    The AT-3 was considerably slower than the PW-9A since the aircraft's weight had not been reduced in proportion to the lesser power. Consequently, the AT-3 trainer was neither as fast nor as maneuverable as it could have been if it had been initially designed for the purpose. Therefore, the Army ordered no production examples of the AT-3. A similar problem was encounterd with the Curtiss AT-4/-5 series of single-seat advanced trainers, which led to an Army decision to convert most of them back to P-1 configuration.

    Armament was two .30 machine guns in the upper fuselage cowling, synchronized to fire through the propeller arc. A heavier .50 machine gun could be substituted for one of the .30 guns. A 244# bomb load could be carried under the fuselage.

PW-9B was the designation given to the last PW-9A [26-375] when it was repowered with a Curtiss D-12D (V-1150-3). Fifteen production PW-9Bs were to have been built, but their designations were changed to PW-9C because of a change in the military specifications under which they were ordered. [26-375] reverted to PW-9A configuration with a D-12C engine when it was delivered to the Army in February 1927.

    PW-9C was the production version of the -9B with modifications to the fuel supply system, a reinforced structure, and bigger wheels on the undercarriage. Outwardly, it was similar to PW-9A except for rearranged flying and landing wire fittings. It was powered by the D-12D engine of the PW-9B. An initial order for 15 examples was placed on June 19, 1926 [26-443/457], followed by an order for 25 more on Aug 18, 1926 [27-128/202]. The first delivery took place on July 9, 1927. Maximum speed was 163 mph at sea level. Weights were 1936# empty, 3170#. gross. Originally the PW-9C had the small unbalanced rudder of the -9A, but that was changed in service (along with the those of the other production models) to the balanced type developed on the experimental Navy FB-3. The last PW-9C [27-202] was delivered with alterations as the first prototype for the PW-9D.

    PW-9D was the final production model. powered by the D-12D engine. It had a newly-designed radiator, landing gear with the same size wheels as the first PW-9s, and wheel brakes. An aerodynamically-balanced rudder of increased area first tested on the Navy FB-3 was adopted as standard. This rudder was later retrofitted to most earlier PW-9s, including Cs already in service.

    Sixteen examples were ordered on Aug 12, 1927 [28-026/041], the first delivery being on Apr 25, 1928. The gross weight of the PW-9D had crawled upward to 3234#, as compared with 2835# for the initial PW-9. Maximum speed was 155mph at sea level, and an altitude of 5000' could be attained in 4 minutes. Service ceiling was 18,230'. The last PW-9D [28-041] was experimentally fitted with a new 600hp Curtiss V-1570-1 Conqueror engine and was redesignated XP-7 under the new system. The last PW-9D in Army service was surveyed in Feb 1934.

Navy versions of the PW-9 series bore the designation FB-1 through FB-5. They were essentially similar to the PW-9, but with various naval equipment such as tailhooks, reinforced fuselages, and beefed-up undercarriages.