In article , Chad Irby
writes
When you break a speed record, one of the requirements is that you do it
in *level flight*.

One could also argue another requirement, is that the 'aircraft'
achieving it, is self-sufficient and is capable of taking off under its
own power. Note the X-1 never held an official air speed record.

Putting a plane into a 40 degree dive kinda takes it out of the running,
especially since some American *prop* planes had probably done it before
1945. From reports, P-38 Lightnings had entered compressibility as far
back as 1941, and some had actually come out of it (not the safest
flight regime, back then).

Spitfires were dived to M.93 after WWII, and were better suited to
speeds in this range than most jets before the Sabre turned up.

--
John