During 1938 the tenth P-36A was used for an experimental engine conversion from a radial to a inline engine, this was designated the Hawk 81 by Curtiss. At the time the inline engine
was favoured after examples of the German Messerschmitt BF 109 and British Supermarine Spitfire and Hawker Hurricane proved successful with this type of engine. Although they did
have their disadvantages, being heavier and having vital systems vulnerable to combat damage, it wasn't until 1941 when the radial engine Focke-Wulf FW 190 went into service that
showed how good radial engines were.
When, on the 14th October 1938, the Hawk 81 flew it was powered by a 1,160-hp Allison V-1710-19 engine, but was basically similar to the P-36A, with the radiator first mounted beneath the
aft fuselage before being placed under the nose. Flying in May 1939, and now designated the XP-40, against other pursuit plane prototypes in competition against each other, the XP-40
was chosen for production by the US Army Air Corps, and on the 27th April 1939, and at the time representing the US Army's largest single order for fighters, 524 P-40s were ordered.
As soon as the first three P-40s rolled of the production line during May 1940 they were sent for service trials. Although these P-40's would be powered by a less powerful supercharged
Allison V-1710-33 engine, and by September 1940 the USAAC had received 200 P-40s. 140 similar Hawk 81A-1 fighters had been ordered by France, but as with the Consolidated PBY Catalina,
none of these reached France before the end of resistance. Instead designated Tomahawk I the aircraft were used by the RAF in Europe where the planes were to prove unsuited for
deployment in Europe, and after replacing Westland Lysanders of No.2 Squadron during August 1941 they were soon relegated to training duties, they also served in North West Africa. No. 112
Squadron was the first RAF overseas Squadron to take delivery of the plane and operating in the Western Desert in a low level ground attack role the plane enjoyed limited success.
The Tomahawk IIA was the next version used by the RAF, the same of the P-40B, with a number of modifications adding to the weight of the plane, such as armour and two more 0.303-in
machine-guns, and as such performance of the plane was reduced. The P-40C was the next variant and as with the P-40B the introduction of two more machine-guns and improved self-sealing
tanks would increase the weight of the plane and thus decrease performance. A number were converted for reconnaissance duties and designated RP-40 and the P-40C that would serve with the
RAF was designated Tomahawk IIB.
A number of P-40s destined for use with the RAF were instead diverted to China to serve with Brigadier General Claire Chennault's 1st American Volunteer Group (AVG), otherwise known as
'The Flying Tigers', who also had the shark tooth design on their planes. This Squadron was working within the Chinese Air Force playing a role in defending China from Japanese
air attacks. The AVG would provide some much needed success over the Imperial Japanese Air Force, and provide America with some much needed morale in the early stages of their entry
into the war.
The P-40D was the product of Curtiss's re-design of the P-40C. A 1,150-hp Allison V-1710-39 engine was installed, armour was added and other changes which would also include the
option of either an external fuel tank or a 500lb bomb and four 0.50-in machine-guns, this was first flown on the 22nd May 1941 and was designated by the RAF Kittyhawk I. However
just over 20 of this type would be built as two more machine-guns were added to bring the total to six and a new designated of P-40E or Kittyhawk IA when used by the RAF, and working
along side the Hawker Hurricane in North Africa the Kittyhawk would find success in the ground-attack role. A number were also converted to two-seat trainers.
Although the Flying Tigers had the famous shark tooth design on their P-40's it was actually No. 112 Squadron of the RAF, based in the Western Desert who introduced the design to the plane.
As the attempts to improve the P-40 had still left it's performance at altitude inadequate for it to fill the role of an effective fighter aircraft, a P-40D was used as the test bed
to see if the introduction of a Rolls-Royce Merlin 28 engine could improve the situation. Flown on the 25th November 1941 the XP-40D showed improvement at high altitude and the
1,300-hp Packard built V-1650-1 Merlin engine would power the P-40F Warhawk. An XP-40G prototype appeared with fuel tank and armament changes, and existing P-40s were modified to
Although the introduction of the Packard built Merlin had improved performance there was a shortage of these engine in the US so the P-40J had a turbo charged Allison engine to
overcome this problem, however this project was abandoned in May 1942. Instead the P40-K would be the next mark to enter service during August 1942, powered by a 1,355-hp Allison
V-1710-73 engine, and later production P-40Ks would have a dorsal fin fitted to help fix the problem of the plane swing on take-off due to the more powerful engine.
The P-40M & P-40L were Curtiss's attempts to give the Warhawk the extra performance needed to become more suited to the fighter role. Despite a weight loss of 250lb on the P-40M
and the introduction of a 1,200-hp Allison V-1710-81 engine on the P-40L hardly any performance increase was achieved.
As 1943 drew to a close it was clear that despite all the attempts to increase the performance of the P-40 that the planes days were numbered, especially as aircraft such as the
North American P-51 Mustang and Republic P-47 Thunderbolt had better performance,
unless a performance increase was found. The P-40N, the last production version, was born,
featuring the same Allison V-1710-81 engine that appeared on the P-40L, the P-40N featured a reduction in armament to four guns mounted in the wings, less fuel capacity and other
minor changes. As a result the maximum speed of the plane was 378 mph, meaning it was the best basic production aircraft of the type. Later on some P-40Ns were modified to become
fighter-bombers with two more machine-guns and a bomb load of upto 1,500lb. This mark would prove to be the most extensively built.
The XP-40Q appeared which identified three prototypes built by Curtiss in it's further efforts to improve the P-40 even more, but despite the introduction of a 1,425-hp Allison
V-1710-121 engine and other improvements it's maximum speed of 422 mph was still below other Allied and Axis fighters in service at the time and development of the plane was stopped.
In comparison the Hawker Tempest Mk I was able to achieve a speed of 466 mph.
Seeing action in the Middle East and the Pacific the Kittyhawk would serve with a number of Air Forces including, the USAAF, RAF and Soviet Union, and by the time production was ended
in December 1944 around 14,00 had been delivered.