Complex sentence: The Mustang was designed in 1940 by North American Aviation (NAA) in response to a requirement of the British Purchasing Commission for license-built Curtiss P-40 fighters.
Split into simpler sentences: The Mustang was designed in 1940 by North American Aviation (NAA) in response to a requirement of the British Purchasing Commission. The Purchasing Commission approached North American Aviation to build under license-built Curtiss P-40 fighters.