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Re-reading the excellent Allan's timeline (Ship shape and Bristol Fashion), the idea for the thread spawned on it's own. So - with technology & aerodynamics of the time, goal is to have a survivable fighter-bomber for 1939 (start of production, initial service use) and 40 (regular to mass production and service use).
All airplane-producing nations qualify, extra points if the A/C is 1-engined, the 2-engined can have another crew member. Performance & payload under 10000 ft are needed, that should quite widen the engine choice. Machine guns, cannons and bombs from the era as wepons - no late-war stuff. Surivability vs. ground fire is wanted, so is realistic way to tackle the tanks.