The P47J. The J was fitted with a high output P&W R-2800-57. This made 2,800 hp @ 2,800 rpm at 35,000 feet. This is in War Emergency Power. The aircraft actually attained 507 mph at an altitude of 34,300 feet. (2,800 hp is 133% of rated power.) At military power (100%), the XP-47J could sustain 470 mph.