The original Curtiss XP-40, ordered July 1937, was converted from the 10th P-36A by replacing the radial engine with a new Allison V-1710-19 engine. It flew for the first time in October 1938. This new liquid-cooled engine fighter had a radiator mounted under the rear fuselage but the prototype XP-40 was later modified and the radiator was moved forward under the engine.

Curtiss XP-40 Warhawk. WRG# 0020977
Curtiss XP-40 Warhawk.
[Source: USAF Photo]

P-40/Tomahawk I
The P-40 (Curtiss Model 81A-1) was the first production variant, 199 built.

One P-40 was modified with a camera installation in the rear fuselage and re-designated P-40A.

P-40B/Tomahawk IIA
The P-40B had extra .30in in (7.62 mm) U.S., or .303 in (7.7 mm) machine guns in the wings and a partially protected fuel system

Curtiss Tomahawk IIA. WRG# 0021381
Curtiss Tomahawk Mk IIA/AH973.
[Source: Mark Allen Collection via the Warbird Information eXchange]

P-40C/Tomahawk IIB
The C model added underbelly drop tank and bomb shackles, self-sealing fuel tanks and other minor revisions, but the extra weight did have a negative impact on aircraft performance. (All versions of the P-40 had a relatively low power-to-weight ratio compared to contemporary fighters.)

Curtiss Tomahawk IIB. WRG# 0021385
Curtiss Tomahawk Mk IIB.
[Source: Mark Allen Collection via the Warbird Information eXchange]

P-40D/Kittyhawk Mk Is
With a new, larger Allison engine, slightly narrower fuselage, redesigned canopy, and improved cockpit, the P-40D eliminated the nose-mounted .50 in (12.7 mm) guns and instead had a pair of .50 in (12.7 mm) guns in each wing. The distinctive chin airscoop grew larger so they could adequately cool the large Allison engine. Less than 50 built.

Curtiss P-40D Warhawk. WRG# 0021380
Curtiss P-40D Warhawk.
[Source: Mark Allen Collection via the Warbird Information eXchange]

P-40E/Kittyhawk IA
The E model was similar in most respects to the P-40D, except for a slightly more powerful engine and an extra .50 in (12.7 mm) gun in each wing, bringing the total to six. Some aircraft also had small underwing bomb shackles. Supplied to the Commonwealth air forces as the Kittyhawk Mk IA. The P-40E was the variant that bore the brunt of air-to-air combat by the type in the key period of early to mid 1942, for example with the first US squadrons to replace the AVG in China (the AVG was already transitioning to this type from the P-40B/C), the type used by the Australians at Milne Bay, by the New Zealand squadrons during most of their air to air combat, and by the RAF/Commonwealth in North Africa as the Kittyhawk IA.

Curtiss P-40E Warhawk. WRG# 0021389
Curtiss P-40E Warhawk.
[Source: Mark Allen Collection via the Warbird Information eXchange]

P-40F and P-40L/Kittyhawk Mk IIA
The P-40F/L, which both featured Packard V-1650 Merlin engine in place of the normal Allison, and thus did not have the carburETOr scoop on top of the nose. Performance for these models at higher altitudes was better than their Allison-engined cousins. The L in some cases also featured a fillet in front of the vertical stabilizer, or a stretched fuselage to compensate for the higher torque. The P-40L was sometimes nicknamed "Gypsy Rose Lee", after a famous stripper of the era, due to its stripped-down condition. Supplied to the Commonwealth air forces under the designation Kittyhawk Mk II, a total of 330 Mk IIs were supplied to the RAF under Lend-Lease. The first 230 aircraft are sometimes known as the Kittyhawk Mk IIA. The P-40F/L was extensively used by U.S. fighter groups operating in the Mediterranean Theater.

Curtiss Kittyhawk MkIIA. WRG# 0021382
Curtiss Kittyhawk MkIIA/FL220 (former US P-40F/41-13698).
[Source: Mark Allen Collection via the Warbird Information eXchange]

P-40 aircraft fitted with the wings of the Tomahawk Mk IIA. A total of 16 aircraft were supplied to the Soviet Union, and the rest to the US Army Air Force. It was later redesignated RP-40G. 43 Built.

The P-40K was an Allison-engined P-40L, with the nose top scoop retained and the Allison configured scoop and cowl flaps. Supplied to the Commonwealth air forces as the Kittyhawk Mk III, it was widely used by US units in the CBI.

Curtiss P-40K Warhawk. WRG# 0021396
Curtiss P-40K Warhawk.
[Source: Mark Allen Collection via the Warbird Information eXchange]

P-40M/Kittyhawk Mk. III.
version generally similar to the P-40K, with a stretched fuselage like the P-40L and powered by an Allison V-1710-81 engine giving better performance at altitude (compared to previous Allison versions). It had some detail improvements and it was characterized by two small air scoops just before the exhaust pipes. Most of them were supplied to Allied countries (mainly UK and USSR), while some others remained in the USA for advanced training. It was also supplied to the Commonwealth air forces as the Kittyhawk Mk. III.

P-40N/Kittyhawk Mk IV
The N Model was manufactured from 1943–44 and was the final production model. The P-40N featured a stretched rear fuselage to counter the torque of the larger, late-war Allison engine, and the rear deck of the cockpit behind the pilot was cut down at a moderate slant to improve rearward visibility. A great deal of work was also done to try and eliminate excess weight to improve the Warhawk's climb rate. Early N production blocks dropped a .50 in (12.7 mm) gun from each wing, bringing the total back to four; later production blocks reintroduced it after complaints from units in the field. Supplied to Commonwealth air forces as the Kittyhawk Mk IV. A total of 553 P-40Ns were acquired by the Royal Australian Air Force, making it the variant most commonly used by the RAAF. Subvariants of the P-40N ranged widely in specialization from stripped down four-gun "hot rods" that could reach the highest top speeds of any production variant of the P-40 (up to 380 mph), to overweight types with all the extras intended for fighter-bombing or even training missions.

Curtiss Kittyhawk MkIIA. WRG# 0021383
P-40N-5-CU Warhawk/42-104888 tramsferred to Royal Air Force as Kittyhawk IV/FT926.
[Source: Mark Allen Collection via the Warbird Information eXchange]

The designation of 1,500 aircraft ordered with V-1650-1 engines, but actually built as the P-40N with V-1710-81 engines.

The XP-40Q had a 4-bladed prop, cut-down rear fuselage and bubble canopy, supercharger, squared-off wingtips and tail surfaces, and improved engine with two-speed supercharger was tested, but its performance was not enough of an improvement to merit production when compared to the contemporary late model P-47Ds and P-51Ds pouring off production lines. The XP-40Q was, however, the fastest of the P-40 series with a top speed of 422 mph (679 km/h) as a result of the introduction of a high-altitude supercharger gear. (No P-40 model with a single-speed supercharger could even approach 400 mph (640 km/h)) With the end of hostilities in Europe, the P-40 came to the end of its front line service.

The designation of P-40F and P-40L aircraft, converted into training aircraft in 1944.

Curtiss P-40L Warhawk/42-10554. WRG# 0021435
Curtiss P-40L Warhawk/42-10554.
[Source: Mark Allen Collection via the Warbird Information eXchange]

Some American P-40s were converted into reconnaissance aircraft.

Designation for aircraft converted into two-seat trainers.

Twin P-40
One of the most unusual concepts, the Twin P-40 was a P-40C outfitted in 1942 with a pair of 1,300 hp (969 kW) Packard V-1650-1 Merlin engines mounted atop the wings, over the main landing gear. Evidence suggests it never made it past the full scale mock up stage.

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