In 1944 Vought converted 3 F4Us as test beds to test R-2800-18W and –16 engines equipped  with high altitude turbo-superchargers. The R-2800-8 engine used in the basic Corsair had a two stage supercharger system. The objective of the proposed change to the turbo-supercharger system was to obtain greater speeds for the Corsair at higher altitudes. During evaluation tests in 1944, the turbo system proved to be faulty and cumbersome and though there was a measurable increase in speed in the 30,000 foot region the project was cancelled. Goodyear was to have built these high altitude turbo-supercharged aircraft as FG-3s. All three test aircraft were returned to the F4U-1 configuration. One of the XF4U-3 aircraft used to test the R-2800-16 engine is sometimes referred to as the XF4U-3B.