On January 25, 1944 Chance Vought Aircraft received from the Navy a letter of intent on a company proposal to build the XF4U-4.  The program began with the modification of two F4U-1 airplanes as prototypes which contained the essentials of a new engine, revised air ducts, large oil coolers and a new air duct at the bottom of the engine cowl to handle increased engine air requirements, and a four-bladed propeller instead of three.  These two airplanes were designated F4U-4X models.  First flight of the F4U-4X was April 19, 1944.

The test pilot reported a markedly increased performance on the first flight. The top speed increased to 450 mph at 26,200 feet compared to the F4U-1D’s 425 mph at 20,000 feet. The rate of climb increased from 3,100 feet per minute to 4,000 feet per minute. The service ceiling increased by almost 5,000 feet to 41,500 feet.

Other changes included in the XF4U-4 were the raising of the cockpit deck to raise the pilot’s legs to decrease the effect of acceleration during combat manueuvers, the repositioning of  instruments and controls, the use of a new bubble canopy and the addition of a new armor-plated seat that provided more pilot protection. 

Fighter-Bomber capabilities were enhanced by installing rocket stations on the wing outer panels. The appearance of the new Corsair remained essentially the same with the long-nose inverted-gull-wing design. In addition to the increased performance the F4U-4B version carried four 20-mm cannons in place of the six 50-caliber machine guns. It could carry rockets on the outer panel wing mounts. Twin pylons located on the wing center section could carry either two 150-gallon drop tanks, two 1,000-pound bombs or two big 11.75-inch rockets.