In 1943, two P-47D-15-RE airframes (serials 42-23297/23298) were selected for testing with the new experimental 2300 hp Chrysler XIV-2220-1 sixteen-cylinder inverted Vee liquid-cooled engine. These aircraft were redesignated XP-47H. The liquid-cooled Chrysler engine with its large under-fuselage radiator radically changed the appearance of the Thunderbolt, and increased overall length to 39 feet 2 inches. With the increased power and improved streamlining, a maximum speed of 490 mph was anticipated.
Although the project was begun in August 1943, the two P-47D-15-RE airframes were not actually converted until 1945. Test flights began on July 26, 1945. One of my sources (Green) says that during flight trails, one of the XP-47Hs actually attained a speed of 490 mph in level flight. However, another one (Wagner) says that the Chrysler engine failed to deliver the promised power output, and that the maximum speed attained during tests was only 414 mph at 30,000 feet, poorer performance than the "stock" P-47D. In any case, the Chrysler XIV-2220 engine never achieved production and the advent of jet propulsion killed any further USAAF interest in the development of even faster piston-engined fighters. Consequently, no further work was undertaken on the XP-47H project.