"On 1 October 1940, the XF4U-1 made a flight from Stratford to Hartford with an average ground speed of 405 mph, the first U.S. fighter to fly faster than 400 mph."
If that was the actual wording of the Vought press release, that's not too misleading. It doesn't claim a world record (it wasn't). It implies that the course was not flown in both directions, a record requirement that would eliminate a beneficial tail wind component*. Moreover, it specifically states that it was a ground speed, not an air speed, which is what really counts in an apples-to-apples comparison of airplane performance. It doesn't describe how, much less how accurately, the beginning and end points of the speed run were determined from an altitude of over 20,000 feet. (At the time, speed records had to be set at a very low altitude. However, top speed for propeller-driven airplanes was attained at altitude. Click
Here for examples.) It also doesn't state the engine rating or horsepower used. The distance, about 50 miles, does limit the amount of altitude that could be exchanged for speed and reduces the impact of an error in determining the beginning and end points.
The Navy's Service Acceptance Trials report provides a more accurate measurement of the XF4U's performance: 371 mph at rated power with the specified useful load. (Click the picture for a readable version.) The Navy did wind it up to 402 mph, but with the "radio mast and antenna not installed, the gun holes and handholds faired." There's no mention of what rpm and manifold pressure were used to attain that speed.
To be fair, the XF4U was powered by the X model of the Pratt & Whitney R-2800 engine. Later production models of the Corsair with more developed and powerful R-2800 engines were capable of exceeding 400 mph in level flight at altitudes above 20,000 feet.
Ron Lewis also notes that the Lockheed P-38 Lightning was the first fighter to exceed 400 mph in level flight, making the XF4U the first single-engine fighter to do so. In a quick interweb search, I didn't find a Lockheed or Army Air Forces claim that the sole XP-38—which first flew on 27 January 1939 and crashed about two weeks later at the end of an attempt to set a transcontinental speed record—reached 400 mph in its brief flight test career. It was reportedly capable of that (click
HERE). The first YP-38 flew on 17 September 1940, about two weeks before the XF4U's "400 mph" flight, a second chance for the Lightning to have beaten the XF4U to that milestone. Again, I didn't find a report of a claim to that effect. Therefore, although the P-38 was appears to be the first U.S. fighter
capable of 400 mph in level flight, Vought might have been correct with respect to the XF4U being the first to actually do it.
* At the same altitude both ways. There was a helicopter speed record set by taking advantage of a tail wind in both directions, but that's a story for another time...
Most of the Navy's jet fighters through the 1950s to a greater or lesser degree were general purpose, in that they could also drop bombs and fire rockets. The F8U was an exception for a few years. It was originally equipped with an integral rocket pack, but this may have been primarily intended as an air-to-air weapon. In any event, it was deleted after the F8U-2 (F-8C).
