To describe today's supercars, we fall back on likening the Koenigseggs and Paganis of the world to jet fighters. The 1953 General Motors Firebird I XP-21 was an actual jet fighter, with four wheels, a tail fin, and a bubble cockpit. The turbine engine spun at up to 26,000 rpm to generate a whopping (for the time) 370 horsepower.