As to float planes...
The world record for speed (internal combustion engine)was set at 440.683 mph by Warrant Officer Agrllo on 23 October 1934. That record still stands.
MACCHI M.C.72
As to the P-38 vs F4U 400mph firet 400+mph U.S.A. aircraft thing,
Maximum speed
XP-38 was 413 mph, all flights prior to crash in February 1939.
YP-38, the Army received its first YP-38 for service trials in March 1940, was 405 mph at 10,000 feet.
Widewing, help! I just can't find the the first 400+ date for the P-38.
The F4U was the first Navy 400+ aircraft, not the first U.S 400+ aircraft.