The Hawk series of fighter aircraft was developed directly from a line of specialized racing planes that the Curtiss Aeroplane and Motor Company of Buffalo and Garden City had built for the Army and Navy between the years 1921 and 1925. The powerplant for the racers was a 435hp Curtiss-developed compact, water-cooled, direct-drive V-12 design. The engine bore the manufacturer's designation of D-12, but in the middle '20s the US military adopted a system in which type and displacement were used as the basis for engine designations. D-12 became the V-1150—V- for the basic Vee design and 1150 for its displacement as measured in cubic inches.

   The first fighter based on the new 435hp Curtiss D-12 originated in 1922 as a private venture by Curtiss. The design was given the company designation of Model 33. Three prototypes were ordered by the USAAS on Apr 27, 1923 as PW-8s [23-1201/1203]. Examples of a basically similar competing Boeing design were also ordered by the Army and given the designation PW-9.

   The designation PW-8 stood for "Pursuit, Water-cooled, Model 8." This Army designation scheme had been introduced in 1920. There were seven separate Pursuit categories, chosen according to the role of the aircraft and the type of engine which powered it—PA (Pursuit, Air-cooled), PG (Pursuit, Ground Attack), PN (Pursuit, Night), PS (Pursuit, Special Alert), PW (Pursuit, Water-cooled), R (Racer), and TP (Two-seat, Pursuit). PW-8 prototypes were redesignated XPW-8 in 1924 when the X-for-experimental prefix was adopted.

The first XPW-8 prototype was delivered to the Army on May 14, 1923. The fuselage was of welded steel tube construction with fabric covering. The undercarriage was a divided-axle design. Wings were all-wood and of a very thin section that required two bays of interplane struts for stiffening. The cooling system for the D-12 consisted of a set of wing-surface radiators that had been pioneered on Curtiss racers in 1922. The radiators were mounted flush with the upper and lower surfaces of the top wing, resulting in an extremely well-streamlined wing surface.

    In the flyoff between XPW-8 and the competing Boeing XPW-9 at McCook Field, the PW-8 proved faster, but the PW-9 was found to be more maneuverable, tougher, and more reliable. The primary problem that the Army found with the PW-8 was in its unique surface radiator cooling system. Although these radiators improved streamlining, they turned out to be a maintenance headache and were prone to constant leaks. In addition, the Army concluded that such a cooling system would probably be extremely vulnerable to damage by gunfire were the Hawk used in combat.

    The second XPW-8 prototype [23-1202] differed from the first in having a divided type of landing gear with reduced drag. The streamlining of the cowling was improved, and strut-connected ailerons and unbalanced elevators were provided. Gross weight increased from 2768# to 3151#.

Although the Army favored the Boeing design, the Curtiss company nevertheless did get an order from the Army for 25 production PW-8 fighters. The order was given to Curtiss in return for the company's agreement to collaborate on a pet scheme of General Billy Mitchell of attempting a coast-to-coast flight across the USA to be completed between dawn and dusk of the same day.

    Prototype XPW-8 [23-1201] was stripped of all military equipment and used in two unsuccessful attempts in July 1923 to cross the USA in a dawn-to-dusk flight, piloted by Lt Russell Maughan. This aircraft was later fitted with a second cockpit, temporarily given a spurious designation of CO-X (Corps Observation, Experimental) and entered in the 1923 Liberty Engine Builders Trophy race for military two-seaters. It was withdrawn before the race because of objections from the Navy.

    The 25 production PW-8s [24-201/225] that were ordered in Sep 1923 began to be delivered to the Army in June 1924. These aircraft were in the configuration of the second XPW-8 [23-1202], which differed from [23-1201] by having a different undercarriage. Most production PW-8s served with the 17th Pursuit Squadron, although several production PW-8s went to McCook Field for experimental work.

    PW-8 maximum speed was 171mph at sea level, cruising 136mph. Initial climb rate was 1830 ft/min. Service ceiling was 20,350'. Range was 544 miles. Armament consisted of a pair of .30 machine guns mounted above the engine, synchronized to fire through the propeller arc. Weights were 2185# empty, 3155# gross.

    On June 23, 1924 [24-204] was finally able to complete the first successful dawn-to-dusk crossing of the USA. The aircraft, piloted by Lt Maughan, took-off from Mitchell Field on Long Island, with refueling stops at Dayton, St Joseph, Missouri, Cheyenne, and Saldura.

The third prototype XPW-8 [23-1203] from the original order had been held back at the factory for installation of a set of single-bay wings. These new wings had heavier spars that produced a stiffer structure, permitting the installation of only a single bay of struts. The new aircraft was assigned the company designation of Model 34. It was delivered in this form to the Army in Sep 1924, and was later redesignated XPW-8A. The troublesome surface radiators of the first two prototypes were replaced by a core-type radiator built flat into the center section of the upper wing panel.

    A modified rudder without balance area was fitted. XPW-8A [23-1203] was entered into the 1924 Pulitzer Trophy race. When modified for the race, the radiator was installed in a "tunnel" underneath the engine, similar to the installation on the Boeing PW-9. In this guise, it was known as XPW-8AA—it came in third.

    The new core-type radiator of XPW-8A proved to be somewhat less temperamental than the surface radiators of the first two XPW-8s, but it was still considered inadequate by the Army. In the meantime, the USAAS had been impressed by the performance at McCook Field of the competing Boeing XPW-9, which was basically similar to the XPW-8, but had tapered wings, and a tunnel radiator underneath the engine. Consequently, the Army asked Curtiss to fit tapered wings and a tunnel radiator to its XPW-8A and resubmit the aircraft for consideration. Curtiss agreed to the changes, and the modified [23-1203] was delivered to the Army in Mar 1925. The changes resulted in a change of designation to XPW-8B.

    The Army was satisfied with the improved XPW-8B and decided on Mar 7, 1925 to give Curtiss a contract for a production series based on this design. In the meantime, in May 1924 the Army had combined its seven separate pursuit category designations into one single category—"P" for pursuit. The first pursuit aircraft ordered by the Army under this new designation scheme were the production versions of XPW-8B, 15 of which were ordered as serial numbers [25-410/424]. These were given the designation P-1, the first entry in the new series.

P-1 (Model 34A) was the first of the Curtiss biplane fighters to carry the name "Hawk", a name which stuck to Curtiss-designed fighters up to and including the P-40 of WW2. The only external difference between the XPW-8B and the P-1 was the addition of an aerodynamic balance to the rudder of the P-1, plus some minor changes to the single-bay struts. These airframes were fitted with the 435hp Curtiss V-1150-1 (D-12C), but were provided with engine mounts that would permit the installation of the larger 500hp Curtiss V-1400. Original plans were for the last five aircraft of the P-1 order to have the V-1400 installed at the factory. Wings were again of wooden construction, but were tapered. Fuselage was of metal tube construction with fabric covering. A 55-gal auxiliary fuel tank could be fitted underneath the belly.

    The first P-1s were delivered to the Army in Aug 1925. Weights were 2058# empty, 2846# gross. Maximum speed was 163mph at sea level, cruising speed was 136mph. The P-1 could climb to 5000' in 3.1 minutes. Service ceiling was 22,500 feet. Range was 325 miles. P-1 was armed with one .50 and one 30 machine gun mounted in the upper fuselage deck and firing through the propeller arc.

    The first P-1 [25-410] was used primarily for test work. It was briefly fitted with an inverted air-cooled Liberty engine and was entered in the 1926 National Air Races. Later it was fitted with an experimental inline, inverted, air-cooled Wright V-1460-3 Tornado and redesignated XP-17.

    The last 5 P-1s that were destined for the larger Curtiss V-1400 were considered sufficiently different that they were redesignated P-2 when they were delivered to the Army. However, the V-1400 proved to be completely unsatisfactory in service, and three of these P-2s [25-421, -422, -424] were converted back to P-1A standards after less than a year of service.

P-1A (Model 34G) was an improved P-1 with the improved D-12C. It was the first of the Hawks to serve in quantity with the USAAC. 25 were ordered in Sep 1925, with deliveries beginning in Apr 1926 [26-276/300]. The fuselage was 3" longer than the P-1, cowling lines were revised, fuel system was changed, and the bomb-release system was improved. In addition, some additional service equipment was provided which increased the weight by some 20 pounds and decreased the top speed slightly. Weights were 204# empty, 2866# gross. Maximum speed was 160mph at sea level, cruising speed was 128mph. P-1A could climb to 5000' in 2.6 minutes. Initial climb rate was 2170 ft/min. Service ceiling was 21,000' and range was 342 miles. P-1A had the same armament as P-1. Three additional P-1As resulted from installation of D-12s in P-2 airframes as described earlier. Only 23 out of the 25 P-1As originally ordered were delivered as such. [26-296] was later modified as the prototype for the XAT-4 trainer, and [26-300] was transformed first into XP-3, then to XP-21 and -21A.

    P-1A [26-295] was modified into an Army racer known as XP-6A #1. The old XPW-8A wings were installed, along with the PW-8-type surface radiators. The new V-1570 Conqueror was installed in a PW-8-type nose cowling and various other minor refinements were made. A really fast aircraft was the result—it won the 1927 National Air Race at a speed of 201mph; however, it was destroyed shortly before the 1928 NARs. XP-1A was applied to a stock P-1A [26-280] diverted to test work. Despite the X-prefix, it was not a prototype.

P-1B was an improved model ordered in Aug 1926 [27-63/87]. Deliveries to USAAC began Oct 28, 1926. The radiator was slightly more rounded and the wheels slightly larger in diameter. The cowling was redesigned. Flares were added for night landings and controls were improved. Equipment changes increased the weight still further, reducing the performance still more. First deliveries to the Army began in Dec 1926. Engine was the 435 hp Curtiss V-1150-3 (D-12D). Weights were 2105#, empty, 2932# gross. Maximum speed was 160mph at sea level, cruising speed was 127mph. Initial climb rate was 1540 ft/min, service ceiling was 21,400', and maximum range was 600 miles. Armament was identical to P-1 and P-1A. The P-1Bs served with squadrons already flying earlier Hawks.

   XP-1B was applied to a couple of stock P-1Bs [27-71, -73], which were used for test work at Wright Field. The latter had machine guns mounted in the wings.

33 improved versions known as P-1C (Model 34O) were ordered in Oct 1928 [29-227/259]. This was the largest Army Hawk order to date. First deliveries to the Army began in Apr 1929. P-1C had larger wheels fitted with brakes. The last 2 P-1Cs were fitted with hydraulic instead of rubber-block shock absorbers. Once again, weight increased and performance decreased. Engine was the 435hp Curtiss V-1150-5 (D-12E). Maximum speed was 154mph at sea level, cruising speed was 124mph. Service ceiling was 20,800'. Empty weight was 2136#, gross weight was 2973#. The P-1C could climb to 5000' in 3.9 minutes. Initial climb rate was 1460 ft/min. Service ceiling was 20,800'. The normal range was 328 miles, with a maximum range of 554 miles.

    P-1C [29-259] was completed as the XP-6B, with a Conqueror engine in place of the D-12, intended for a long-range flight from New York to Alaska. However, the XP-6B crashed short of its goal, and was shipped back to the USA for repair and subsequent test work.

    XP-1C was the designation applied to P-1C [29-238] diverted to test work, fitted with an experimental Heinrich radiator and Prestone cooling system. Despite the X-prefix, it was not a prototype.

    In 1924 the Army decided it would be a good idea to equip some of its up-to-date pursuit designs with lower-powered engines and use them as advanced trainers. These advanced trainers were all unarmed. However, the concept was not very successful. Since the trainers used the same airframes as did the fighters, the lower-powered trainers were vastly overstressed for their missions, and were overweight for their power and had very poor performances. After a short service, these advanced trainers were converted to full fighter configuration, provided with armament, and were retrofitted with D-12 engines. These converted trainers were then given pursuit designations P-1D through -1F.

The first Curtiss advanced trainer prototype had been created by fitting P-1A [26-296] with the liquid-cooled 180hp Wright-Hispano E. It was delivered to the Army in July 1926 under the designation XAT-4—"AT" stood for "Advanced Trainer."

    AT-4 was the production version of XAT-4. 40 were ordered in Oct 1926 [27-88/97, -213/242]. All were powered by the Wright-Hispano E (V-720). Maximum speed was 133mph at sea level, cruising speed was 107mph. Initial climb was 950 ft/min. Range was 535 miles. Weights were 1847# empty, 2484# gross. 35 AT-4s were eventually converted back to fighter configuration by the fitting of the Curtiss V-1150-3 (D-12D) and the a single .30 machine gun. These converted aircraft were assigned the designation P-1D.

    The last 5 airframes of the AT-4 order [27-238/242] were completed as AT-5s, with the 220hp Wright J-5 (R-970-1) Whirlwind radial in place of the Wright-Hispano liquid-cooled engine. This engine was considerably lighter than the Wright-Hispano, but the disadvantage of lower power was still there. Maximum speed was 125mph at sea level, cruising speed was 100mph. Initial climb was 1096 ft/min. Range was 488 miles. Weights were 1802# empty, 2445# gross. The AT-5s were later redesignated P-1E when they were repowered with 435hp D-12Ds and fitted with a single .30 machine gun. Both the P-1D and the -1E served with the 43rd School Squadron at Kelly Field.

    AT-5A (Model 34M) was an improved AT-5, with the longer fuselage and other structural improvements of the P-1A. 31 examples were ordered by the Army on July 30, 1927 [28-42/72]. In 1929 all AT-5As were converted to fighter configuration with the switch to a 435hp D-12D and the addition of armament, and redesignated P-1F. One other P-1F [28-189] was obtained by converting an XP-21, which in turn had earlier been converted from a P-3A.

There were only a few export sales of the P-1 Hawk. Four P-1s were sold to Bolivia. Eight export models of the P-1A were sold to Chile in 1926. One example was sold to Japan in 1927. Eight P-1Bs were exported to Chile in 1927, and some units are believed to have been built there.

    With each successive variant, the weight of the P-1 fighter increased, leading to a gradual falloff in the top speed and in the climbing performance. P-1s were flown by 27th and 94th Pursuit Squadrons of the 1st Pursuit Group based at Selfridge Field in Michigan and later by the 17th Squadron, who kept them in service until 1930 until they were replaced by later types. I don't think they ever fired a shot in the defense of American territory.