The P-36 and P-40 both had a serious flaw, that being the lack of high altitude performance of the engines. They never did figure this out. The airframe was too heavy until two stage supercharged RR Merlin engines were available, and these were put on the (faster and longer-ranged) P-51 by the British, and later in an official capacity by NAA as we all know. Because of the relatively low altitude performance ceiling of the engines, both speed and range of the P-36 and P-40 series were limited (since the highest speed and the best cruise efficiency were reached at relatively high altitude, generally speaking) and it limited their effectiveness as an interceptor against Operational or Strategic bombers - that is to say specifically, against high-flying multi-engined level-bombers.
This made Generals, military planners and strategists deeply dislike the Curtiss Hawk family of fighters. The P-36 was not suitable for the defense of England during the BoB, because it wouldn't have been good fighting at 20,000 feet or more where the German medium bombers and their Bf 109 escorts were flying. The P-40 also had problems early on against Japanese long range medium bombers (G3M, G4M, Ki-21 etc.) in the Pacific though they were able to get around the performance ceiling issues by using clever tactics. It wouldn't be good for escorting bombers on those kinds of raids either, and had very limited Strategic value.
However, in Tactical combat, the P-36 and P-40 both rose to the occasion far better than expected (or that they should have a right to do on paper, so to speak) and continued to be effective throughout the War.
The P-36 was the most effective Allied fighter in the Battle of France - and this includes the Hurricane and Spitfire.
The P-40 was the most effective Allied fighter in the early period (roughly the first year) of the Western Desert in Commonwealth use. It was the best hands down in the CBI, and along with the Wildcat, was the best Allied fighter in the Pacific in the initial crucial stages. It was also one of the most effective fighters available to the Soviets in the pivotal and desperate year of 1942, though the Soviets had a lot of maintenance problems with them.
By 1942, the Hurricane, with which the P-40 is so often compared, had clearly slipped a notch. The British and Commonwealth had downgraded it beneath the P-40, the latter often providing escort for the former after 4 RAF, 2 RAAF and 4 SAAF Fighter Squadrons were converted over to the Tomahawk. The Soviets hated the Hurricane and phased it out of use in 1942, whereas they were still using P-40s in the front line for another year, and in the Baltic and PVO (air defense) squadrons until the end of the War.
The P-40 was continuously improved through 1943, including the use of License-built Merlin XX engines and various weight reducing programs, but even the Merlins were not high altitude engines and even with significantly boosted horsepower from the Merlis or much more powerful Allisons (V-1710-73 for example), the P-40 never made the magic cutoff line of 400 mph, and never became the Strategic Escort or high altitude interceptor that the Generals wanted. Curtiss Aircraft company arguably never made another successful major combat aircraft after the P-40 and had a series of ever more egregious and embarrassing scandals which made the War Dept dislike them even more.
However, the improvements, even though they appeared marginal on paper, continued to enhance the capabilities of the P-40 sufficiently to keep it in the game, and not just as a fighter bomber as is so often claimed in various summaries. The extremely maneuverable fighter had a very high dive speed and good high speed handling, enabling pilots to escape or disengage from combat when they needed to, a critical ingredient for a successful fighter which the Hurricane, for example, never managed, and which the Wildcat didn't really manage when facing German or Italian fighters. It was also able to outmaneuver enemy fighters in the MTO that attempted to dogfight, and catch enemy planes that tried to dive away.
The P-39, the ostensibly much more modern stablemate of the P-40, never worked out in US or Commonwealth use. Newer American types like the P-38 and P-47 were late to the game and had extended teething problems (the P-47 having to go from P-35, to P-43, through a series of early P-47 models before reaching maturity). Gnashing their teeth, the US Generals were forced to equip 5 Fighter Groups with P-40F/L for use in North Africa and Italy (
57th,
33rd,
79th,
324th, and
325th Fighter Groups) plus the
99th FS of the Tuskegee airmen. During and right after the War they were often portrayed as doing poorly, but their records were actually very good, better than most MTO P-38 squadrons and competitive with the Spitfire V squadrons in terms of victories vs. losses.
The P-40 itself was able to remain competitive, in the sense that they could still fly 'armed recon', fighter sweeps, escort Tactical bombers (frequently A-20s and B-25s) and strafing runs on enemy fighter bases with a reasonable probability of survival all over the world, well into 1944. In the CBI this continued into 1945. The reason was the maneuverability and the means of disengaging, as well as the oft mentioned ruggedness and firepower of the type. Those qualities tend to be overemphasized though since with the increasing firepower of late-war fighters meant that no amount of armor or sturdy construction could be counted on to save you in a fight, at best they increased the chances of survival incrementally. The real reason P-40 pilots so often survived combat was the maneuverability and high dive speed.
Even in 1943 and 1944 you get experienced pilots flying P-40s shooting down Bf 109s, MC 205s, Fw 190s and Ki43s and A6Ms in large numbers, at favorable rates.
Details I have seen posted in other threads on this very forum show that P-40 pilots were actually holding their own very well even against late war types like the Ki-84 and Ki 61, and planes (like Zeros) flown by the most experienced crews.
The P-40 may not have been the most underrated but it's still very negative reputation is at odds with it's admittedly uneven but often very good combat record.
S