Evolved from Seversky P-35 and Republic P-43 Lancer. Version which eventually became P-47 originally proposed as lightweight fighter with liquid-cooled engine. Alternate proposal using then-most powerful available engine, P&W Double Wasp 2000 hp radial, preferred by USAAC. Nicknamed "Jug", was heaviest (about 18,000 lbs GTOW) single-seat, single-engine fighter of WWI and was only radial-engine fighter produced in quantity for USAAF. D produced in greatest numbers; other countries, including 195 to Russia, also used them. D had improved engines giving better performance at high altitude and in emergencies, and also a notable improvement in weaponry. Bubble canopy first appeared on D-25 series and all later versions. N model designed especially to meet demands in Pacific operations; had more powerful armament, 2800 hp engine, and increased range with sufficient fuel for missions up to 12 hrs.