whoops I stand corrected:
http://www.p-47thunderbolt.com/Another version of the Thunderbolt was the P-47J. This model introduced a closely cowled fan-cooled R-2800-61 engine rated at 2,100 hp, and provided with a wing of lighter weight that was fitted with 6 .50 in machine guns. The J model Thunderbolt first flew on November 26, 1943 and was the first pison engined machine to exceed a speed of 500 mph in level flight. despite the excellent performance the project was abandoned to favor another variant(Xp-47M).
Although sometimes experimental prototypes do things that production models never can, i.e. special souped-up Bf-109's were reaching 440 mph in the late 30's.
