Boeing XP-26

Last revised June 12, 1998




The Boeing P-26 was an interesting mixture of the old and the new. It was the first production monoplane fighter and the first all-metal fighter to serve with the USAAC. It was also the last USAAC fighter to have externally-braced wings, an open cockpit, and non-retractable undercarriage. For Boeing, it was also their last production fighter, and it brought to an end the fifteen-year period during which Boeing dominated the market for fighters for both the Navy and the USAAC.

In the early 1930s, the USAAC was faced with a new generation of monoplane bombers which were faster than its current stable of biplane fighters. In September 1931, the USAAC approached Boeing engineers with a requirement for a monoplane fighter that would be faster than the most up-to-date bombers that were flying at the time. Initial work on the project was begun at Boeing expense that very month. The company designation for the project was Model 248. On December 5, 1931, Boeing and USAAC signed a bailment contract for three prototypes for a monoplane fighter. The experimental designation of XP-936 was applied. The X-series of "project numbers" were allocated at the Army's test center at Wright Field during 1930-34 to aircraft built by private companies without government funding but with Army=owned engines and other equipment on loan. Under the terms of the agreement, Boeing would provide the airframe, but the Army would provide the engines and the instruments. Boeing would retain ownership of the airframes.

The design progressed rapidly, and actual construction of the Model 248 began in January of 1932. The design which finally emerged was a compromise between the advanced ideas of Boeing and the conservative and cautious approach favored by the USAAC. It was a look forward and a look backward.

The Model 248 (XP-936) was an all-metal low-wing monoplane with wire-braced wings (a cantilever wing was rejected as not being strong enough for a fighter). The wing was of low aspect ratio with a thin section. The wing had two main spars, built up of sheet and angle duralumin. The wing supported dural ribs, to which was riveted a series of closely-spaced spanwise stringers which supported the metal skin. The use of external wire bracing for the wing would at first sight seem to have been a retrograde step, however the use of wire bracing for the wing permitted a lighter wing structure. Even so, wires produced less drag than would have been produced by the use of rigid struts. The horizontal tailplane, unlike the wing, was of fully cantilever construction.

The fixed, non-retractable undercarriage would also seem to have been a retrograde step. The fixed undercarriage did indeed add extra drag, but it had an advantage in reducing weight and structural complexity and it provided a structurally efficient low anchor point for the flying wires. The rear portion of each undercarriage unit consisted of an inverted tubular bipod fastened to the front and rear wing spars. The flying wires were attached to the apex, and the wheel pivoted about the apex on an arm, with the landing loads being absorbed through a shock absorber strut connecting the wheel axle to the front spar. The entire lower portion of the undercarriage was enclosed by streamlined wheel fairings. A tailwheel took the place of the traditional skid. Wheel brakes were provided.

The fuselage was of a semi-monocoque type with aluminum bulkheads, hat-section longerons, skin stiffeners and skin. The aircraft was to be powered by the well-proven Pratt and Whitney R-1340-9 nine-cylinder air-cooled radial (no experimenting with untried engines this time!). A Townend-type drag ring was fitted around the engine. Armament was provided by one 0.50-in M2 and one 0.30-in M1 machine guns, or two 0.30-in M1 machine guns, mounted in the fuselage sides on the cockpit floor and firing through the engine cylinder banks. Each gun could carry up to 200 rounds. Under-fuselage A-3 bomb racks could be fitted which were capable of carrying five 30-lb bombs, two 122 lb bombs, or two parachute flares. A C-3 tubular gunsight was mounted ahead of the windshield, and a G-4 camera gun could be mounted externally above the right-side wing root.

The pilot sat high relative to the cockpit rim and had fairly good visibility in all directions except to the rear. However, the Townend ring impeded visibility while taxiing on the ground, so it was standard practice to do an S-turn while taxiing.

The first XP-936 made its maiden flight on March 20, 1932, test pilot Les Tower being at the controls. The design had progressed from drawing board to first flight in only 9 weeks, which was a remarkable feat for a major USAAC fighter project. It was sent to Wright Field for Army evaluation on April 25. The second XP-936 had been completed and delivered to Wright Field before the first machine had made its maiden flight. It was later flown to Anacostia, Maryland, for demonstration to the Navy. It was then returned to Wright Field, where it underwent a series of static tests by the Army. On April 25, the third XP-936 was sent to Selfridge Field, Michigan, for service testing with the 1st Pursuit Group. While undergoing tests at Wright Field, the XP-936 weighted 2119 lbs empty and 2789 lbs loaded. Maximum speed was 227 mph at 10,000 feet, 210 mph at 20,000 feet, and 174 mph at 27,800 feet, the aircraft's service ceiling was 28,900 feet. Initial climb rate was 2260 feet per minute. Range was 758 miles.

At both Wright and Selfridge Fields, the Boeing-owned ships were flown by Army pilots. It is interesting to compare the general performance of the XP-936 with that of the P-12F biplane, which was still in production at the time the XP-936 was delivered. The XP-936 had a slightly later version of the same engine that powered the P-12F, and the engine gave the monoplane only a 20 hp power advantage over its biplane predecessor. The XP-936 was only 39 lbs heavier than the P-12F but was 27 mph faster and outclimbed it by a rate of 476 ft/min. However, the P-26 fell 800 feet short of the biplane's absolute ceiling. The XP-936 outperformed many Air Corps warplanes, but was slower than the Martin B-10 monoplane which was then being proposed for the Air Corps.

The Army bought the three XP-936s on June 15, 1932, assigning them the designation XP-26. Serials were 32-412/414. The designation was soon changed to the service test designation of Y1P-26 and eventually to plain P-26. However, these planes were used only for various test purposes and never entered squadron service with the USAAC.

Sources:

  1. Boeing Aircraft since 1916, Peter M. Bowers, Naval Institute Press, 1989.

  2. "The Boeing P-26A", Peter M. Bowers, in "Aircraft in Profile", Doubleday, 1969.

  3. United States Military Aircraft Since 1909, Gordon Swanborough and Peter Bowers, Smithsonian Institution Press, 1989.

  4. The American Fighter, Enzo Angellucci and Peter Bowers, Orion Books, 1987.

  5. Boeing P-26 Peashooter, Robert F. Dorr, Air International, Vol 48 No 4, p 239 (1995).