The Curtiss P-40 was the only USAAF fighter available in quantity when the US entered WWII in 1941. By that time, the aircraft was almost obsolete compared to its adversaries, but soldiered on throughout the war as various improvements were made in its design. The P-40M and N variants were the last production models and were developed in an attempt to overcome some of the performance deficencies of the aircraft. The P-40N in particular underwent a major program of weight reduction, and as a result was the fastest and most agile of the P-40 series. The N-model also featured improved rear visibility through a redesigned rear canopy. Both the M and N models served with the USAAF and its allies in WWII: Great Britain, Australia, New Zealand and South Africa.