Stand corrected Mr Crun, the Report by RB of that flight clearly states that "the dive was carried out at 30 degrees and at 0.98 the nose of the aircraft was pushed down further", on the same flight the aircraft was accelerated to 0.9 in level flight.
The Bell X 1 aircraft could achieve Mach 1 plus in straight and level flight, please put my sloppy post down to excessive xmas beer.
Best regards,
om15