Curtiss P-40 Warhawk variants
The Curtiss P-40 Warhawk was a WWII fighter aircraft that was developed from the P-36 Hawk, via the P-37. Many variants were built, some in large numbers, under names including the Hawk, Tomahawk and Kittyhawk. Allison-engined Model 75. XP-37. In early 1937, after realizing the Hawk 75 was inferior to more modern European designs, the USAAC ordered one P-36 to be modified with an Allison V-1710 inline engine. The prototype Hawk was fitted with a turbo-supercharged 1,150 hp (860 kW) Allison V-1710-11 engine as the XP-37 (company designation Model 75I). The cockpit was  moved back towards the tail to make room for the massive supercharger, and the engine was cooled by two radiators on either side of the nose. Armament was one .30 M1919 Browning MG and one .50 M2 Browning MG mounted in the nose. The XP-37 was plagued with supercharger and visibility problems. YP-37. A further 13 Model 75Is were ordered in 1938 under the designation YP-37. These differed from the XP-37 in having a V-1710-21 with a more reliable supercharger and an extended nose. The project was cancelled after continued supercharger and visibility problems. XP-40. In 1937, the 10th  P-36A was fitted with a 1,150 hp (860 kW) V-1710-19. Unlike the Model 75I, the resulting XP-40 (Model 75P) did not have a turbo-supercharger, thus the cockpit was not moved back, and the radiator was moved to the ventral position. Later the landing gear was redesigned and the radiator was moved under the nose. Armament was two .50 M2 Brownings mounted in the nose. The new engine conferred a 50 mph speed advantage over the already popular Hawk, and export orders from England and France came in quickly. In April 1939, the United States Army Air Corps also  put in a domestic order for 524 Model 75Ps, which was the largest single order for a US fighter aircraft at the time. Model 81. P-40. The production P-40 (Model 81A) were nearly identical to the XP-40, but was built with a 1,040 hp (780 kW) V-1710-33s and one .30 M1919 Browning in each wing. The company designation was changed to Model 81 due to the extensive changes from the standard Model 75. France, who was a large operator of the P-36, was interested in this fighter and ordered 140 aircraft as Hawk 81A-1s. However, following the 1940 French  Armistice the Royal Air Force acquired these aircraft as Tomahawk Mk.Is. This variant was not considered combat-ready, as they lacked heavy armament and armor, but as there was a shortage of decent fighter aircraft after the Battle of Britain, the RAF pressed these into service for use in North Africa anyway. P-40B. Of the remaining 324 aircraft of the initial order, 131 were built as P-40Bs. Though strongly built, the first P-40 variant was poorly protected and lacked armor and self sealing tanks. This was partly rectified with the P-40B, which had additional armor behind the cockpit, but  fuel system and control line vulnerability remained a problem to some extent with all Tomahawk types. The P-40B also had an additional .30 MG in each wing. P-40C. The last 193 aircraft of the original P-40 order were completed as P-40Cs. In an attempt to further rectify the problem of poor protection, the P-40C was given self-sealing fuel tanks. The obsolete SCR-283 radio of the earlier P-40s was replaced with an SCR-274N and provisions were made for a 52 gal drop tank. The latter change increased the combat radius dramatically, and was a standard feature in all subsequent P-40s . Model 87. P-40D. Soon after P-40 production started, Curtiss began development of its intended successor, the XP-46. This aircraft was based on the P-40, but was an almost entirely different aircraft. While retaining the rear fuselage of the P-40, the XP-46 had a new wing with wider track landing gear. The nose was redesigned too, as it housed a new 1,150 hp (860 kW) V-1710-39 engine. This "F-series" engine differed from the "C-series" engine of the Model 81 in having a more compact external spur gear-type reduction gear box. The production version of the P-46 was  to have four .30 MGs in each wing and two .50 MGs in the nose for a total of ten guns. This would have been the heaviest armament for a US fighter at the time. Both the USAAF and RAF placed orders for this aircraft, with the latter naming it "Kittyhawk". Due to delays in the P-46 program, the USAAF asked Curtiss to prioritize development of an improved P-40. Curtiss did so, and reworked the P-40 to accommodate the V-1710-39 of the XP-46. The resulting P-40D (Model 87A) had a shorter nose with a larger radiator, four .50 Brownings  in place of the .30 units, a revised windscreen, and provisions for two 20 mm cannons (one in each wing, never used). The nose guns were deleted as there wasn't room for them in the final design. Upon testing both the P-40D and XP-46 prototypes, the USAAF found that the XP-46 offered no significant improvement over the P-40C, and was inferior to the P-40D. Both the USAAF and RAF cancelled their orders for the P-46 and the name "Kittyhawk" was given to the P-40D. P-40E. Starting with the 24th Model 87, an additional .50 MG was added to each  wing and the carburetor intake was moved forward 6 in. Although these changes were relatively minor, this new variant was given the designation P-40E. When the P-40D and E went into service, several problems were discovered. When maneuvering in high G turns the guns would often jam due to the way the ammunition was stored. Another problem was that engine and trim management were both somewhat complex and taxing with earlier P-40's required strong rudder pressure to offset engine torque and frequent trim adjustments were needed during rapid speed changes. Both these problems were remedied sometime late in production  by enlarging the vertical stabilizer and changing how the ammunition was stored. P-40's were more a powerful, faster-flying aircraft than the primary and advanced trainers most pre-war and early-war Allied pilots were familiar with, and transition training was often inadequate or neglected altogether in the early years of the war. The landing gear was also more narrow and not as strong as in fixed gear aircraft (like the Gladiator) or on some other retractable gear fighters such as the Hurricane. As a result novice pilots had a hard time adjusting to the new fighter and there  were many accidents on landing and takeoff in the early years of the war, with both Tomahawk and Kittyhawk types. Therefore, two P-40Es were fitted with a second seat to be used as trainer aircraft under the designation P-40ES. With these aircraft and improved training techniques these problems subsided. P-40J. The main problem with the P-40 was its effective altitude ceiling of about 12,000 feet. Above that altitude the single-stage Allison V-1710 engine started to perform poorly. As a result, unless combat was taking place at low altitude, P-40 pilots often faced attack from above in the opening stages  of an interception, a chronic problem which cost many lives. In response, Curtiss proposed the P-40J, which was basically a P-40E with a turbo-supercharged V-1710. Although a good idea on paper, there were many problems with fitting a turbo-supercharged V-1710 into the P-40. One of the main problems was the size of the turbocharger, which would not fit in the standard P-40 airframe. On top of that the intended engines were reserved for P-38 production. In May 1942 the project was cancelled without anything being built. P-40F. Another solution to the high altitude performance problem was to fit  the P-40 with a supercharged Rolls-Royce Merlin engine. This idea came from the British, as they noticed that their Allison powered Kittyhawks and Mustangs had poor performance at high altitudes, while their own Merlin powered Hurricanes and Spitfires did not have the same problems. Curtiss fitted the second P-40D with a 1,300 hp (969 kW) Merlin 28. Production aircraft had the American-made 1,390 hp (1,040 kW) Packard V-1650-1 Merlin. The resulting P-40F (Model 87B) was the first variant to carry the "Warhawk" name. Along with the added power of the Merlin engine came a decrease in directional  stability. Curtiss attempted to fix this by fitting a dorsal fillet to the tail of a single P-40F; however, this was not adopted into production. Starting with the P-40F-5, the tail was lengthened by about 20 in. Although the P-40F was superior to the Allison powered P-40s, there was a shortage of Merlin engines due to the vast number of aircraft that used them. Parts for these engines were becoming scarce, and maintenance became an issue. As a result, at least 70 P-40Fs were re-engined with V-1710-81s of 1,360 hp. These aircraft became known as P-40R-1s. P-40K. The  P-40K was intended to be the last P-40 production variant before replacement by the P-60, and only 600 were ordered by the USAAF to supply to China. However, with the cancellation of the P-60 the attack on Pearl Harbor led to this order being increased to 1,300 aircraft. A continuation of the Allison-powered Warhawk, the K was similar to the P-40E, but was powered by a 1,325 hp V-1710-73. It also featured improved machine gun ammunition storage reducing gun stoppages. These were the heaviest P-40 variants, but the extra horsepower on the P-40K gave it good  performance particularly at low altitude (noticeably better than the P-40E). As with the P-40F, the increase in power led to decreased directional stability, but Curtiss predicted this and incorporated an enlarged vertical stabilizer to early P-40Ks. On the K-10 sub-variant onward, this was replaced with the lengthened tail of the P-40F-5. This feature was standard on all subsequent Warhawks. P-40L. A continued development of the Merlin powered Warhawk, the P-40L was a lightened version of the P-40F. Many weight saving changes were made to the aircraft, including removal of armor plating and reduction of rounds per gun. On the  P-40L-5, weight was further reduced by removing two of the guns and reducing the internal fuel capacity from 157 gal. to 120 gal. As with the P-40F, at least 53 P-40Ls were re-engined with V-1710-81s. P-40M. Due to the shortage of Merlins, development of the Allison powered Warhawk was again continued. The P-40K airframe was given a 1,360 hp V-1710-81 with a cooling grill forward of the exhaust stubs. The P-40M was supposed to be a purely export variant of the K, although many ended up in USAAF units. In RAF service the aircraft was  named Kittyhawk Mk.III, the same as the P-40K, which can cause some confusion. P-40N. The P-40N was the most produced variant of the Warhawk, with 5220 aircraft built. In an attempt to increase performance, Curtiss lightened the P-40M by introducing a lightweight structure, lighter, smaller-diameter undercarriage wheels, removing two of the guns, and installing aluminum radiators and oil coolers. Head armor was also re-introduced. With these changes, the P-40N-1 (Model 87V) was the fastest production Warhawk, reaching a speed of 378 mph below 12,000 feet. Starting with the P-40N-5 (Model 87W), the canopy was redesigned  to give the pilot a better field of vision. This variant also reverted to the six gun wing and one rack was added to each wing, these could carry either bombs or drop tanks. A more powerful V-1710-99 engine was introduced on the N-20, and a further improved V-1710-115 was introduced on the N-40. Curtiss attempted to further improve visibility and fitted one P-40N with a bubble canopy; this feature never made it to production. P-40P. The P-40P was a planned variant of the P-40N with a Merlin engine. The project was cancelled due to a shortage of  Merlins and the aircraft were delivered as P-40Ns. P-40Q. In 1944, Curtiss attempted to bring the Warhawk to the standards of more modern fighters such as the North American P-51 Mustang. To do so, Curtiss installed a 1,425 hp water injected V-1710-121 into the Hawk 87 airframe. The resulting aircraft became the fastest P-40 model at 422 mph. Even with these modifications, the P-40Q (Model 87X) was still inferior to modern fighters and the project was cancelled. Twin P-40. A single photo exists of a P-40 with two Merlin engines mounted on top of the wings over the landing  gear. The apparent serial number, 41-13456, belongs to a P-40C, however, the canopy is from an early Hawk 87.