The task is to find the way(s) that British/Allied war effort gets twice the number of Supermarine Spitfires, from 1938 to 1945, vs. what was produced in the same time. People can kill off one or more OTL designs in order to pump up the number of Spitfires. Engines, guns and other bits & pieces as available on the OTL, the types that were more or less available for the British/Allies (eg. no restriction to just Merlin and Griffon, but no BMW 801 for example). The number can includes Seafires. Ditto for improved Spitfires.
What the Allies can gain with double the number, and presumably improved capabilities of the fighter? Other butterflies?