Design & Development

On 14 October 1938, Curtiss test pilot Edward Elliott flew the prototype XP-40, on its first flight in Buffalo. The XP-40 was the 10th production Curtiss P-36 Hawk, with its Pratt & Whitney R-1830 (Twin Wasp) 14-cylinder air-cooled radial engine replaced at the direction of Chief Engineer Don R. Berlin by a liquid-cooled, supercharged Allison V-1710 V-12 engine. The first prototype placed the glycol coolant radiator in an underbelly position on the fighter, just aft of the wing's trailing edge. USAAC Fighter Projects Officer Lieutenant Benjamin S. Kelsey flew this prototype some 300 miles in 57 minutes, approximately 315 miles per hour (507 km/h). Hiding his disappointment, he told reporters that future versions would likely go 100 miles per hour (160 km/h) faster. Kelsey was interested in the Allison engine because it was sturdy and dependable, and it had a smooth, predictable power curve. The V-12 engine offered as much power as a radial engine but had a smaller frontal area and allowed a more streamlined cowl than an aircraft with radial engines, promising a theoretical 5% increase in top speed.

Curtiss P-40 Warhawk. WRG# 0021412
A group of P-40 Warhawks.
[Source: Mark Allen Collection via the Warbird Information eXchange]

Curtiss engineers worked to improve the XP-40's speed by moving the radiator forward in steps. Seeing little gain, Kelsey ordered the aircraft to be evaluated in a NACA wind tunnel to identify solutions for better aerodynamic qualities. From 28 March to 11 April 1939, the prototype was studied by NACA. Based on the data obtained, Curtiss moved the glycol coolant radiator forward to the chin; its new air scoop also accommodated the oil cooler air intake. Other improvements to the landing gear doors and the exhaust manifold combined to give performance that was satisfactory to the USAAC. Without beneficial tail winds, Kelsey flew the XP-40 from Wright Field back to Curtiss's plant in Buffalo at an average speed of 354 mph (570 km/h). Further tests in December 1939 proved the fighter could reach 366 mph (589 km/h).

An unusual production feature was a special truck rig to speed delivery at the main Curtiss plant in Buffalo, New York. The rig moved the newly built P-40s in two main components, the main wing and the fuselage, the eight miles from the plant to the airport where the two units were mated for flight and delivery.

Performance characteristics
The P-40 had good agility, especially at high speed and at medium-to-low altitude. It was one of the tightest-turning monoplane fighters of the war, although at lower speeds it could not out-turn the extremely maneuverable Japanese fighters such as the A6M Zero and Nakajima Ki-43 "Oscar".

Allison V-1710 engines produced about 1,040 hp (780 kW) at sea level and at 14,000 ft (4,300 m): not powerful by the standards of the time and the early P-40 variants' top speeds were unimpressive. Also, the single-stage, single-speed supercharger meant that the P-40 could not compete with contemporary designs as a high-altitude fighter. Later versions, with 1,200 hp (890 kW) Allisons or more powerful 1,400 hp Packard Merlin engines were more capable. Climb performance was fair to poor, depending on the subtype. Dive acceleration was good and dive speed was excellent. The highest-scoring P-40 ace, Clive Caldwell (RAAF), who claimed 22 of his 28½ kills in the type, said that the P-40 had "almost no vices", although "it was a little difficult to control in terminal velocity". Caldwell added that the P-40 was "faster downhill than almost any other aeroplane with a propeller."

The P-40 tolerated harsh conditions in the widest possible variety of climates. It was a semi-modular design and thus easy to maintain in the field. It lacked innovations of the time, such as boosted ailerons or automatic leading edge slats, but it had a strong structure including a five-spar wing, which enabled P-40s to survive some midair collisions: both accidental impacts and intentional ramming attacks against enemy aircraft were occasionally recorded as victories by the Desert Air Force and Soviet Air Forces. Caldwell said P-40s "would take a tremendous amount of punishment, violent aerobatics as well as enemy action." Operational range was good by early war standards, and was almost double that of the Supermarine Spitfire or Messerschmitt Bf 109, although it was inferior to the Mitsubishi A6M Zero, Nakajima Ki-43 and Lockheed P-38 Lightning.

Caldwell found the P-40C Tomahawk's armament of two .50 in (12.7 mm) Browning AN/M2 "light-barrel" dorsal nose-mount synchronized machine guns and two .303 Browning machine guns in each wing to be inadequate. This was rectified with the P-40E Kittyhawk, which abandoned the synchronized gun mounts and instead had three .50 in (12.7 mm) guns in each wing, although Caldwell preferred the Tomahawk in other respects. It had armor around the engine and the cockpit, which enabled it to withstand considerable damage. This was one of the characteristics that allowed Allied pilots in Asia and the Pacific to attack Japanese fighters head on, rather than try to out-turn and out-climb their opponents. Late-model P-40s were regarded as well armored. Visibility was adequate, although hampered by an overly complex windscreen frame, and completely blocked to the rear in early models due to the raised turtledeck. Poor ground visibility and the relatively narrow landing gear track led to many losses due to accidents on the ground.

War Planes of the Second World War, Fighters, Volume Four, William Green, Doubleday, 1964.