The North American Mustang was an aircraft type with two sides to its story. The widespread fame of Merlin-engined examples as high-altitude escort fighters has led to their Allison-engined predecessors being largely forgotten and often unjustly maligned. This viewpoint is completely wrong and ignores the facts behind the creation of the Mustang in the first place, when high-altitude heavy bomber escort work had not even been considered.
Despite Britain’s ongoing rearmament as war approached during 1939, the country was still short of many necessary components. RAF units were increasingly converting to the excellent Supermarine Spitfires and Hawker Hurricanes that would ultimately win the Battle of Britain in 1940. However, British requirements were many, and certainly more combat aircraft than were available from British factories were going to be needed then and in the following years.