Yes, I suppose part built aircraft count too, in which case the M.52 is a good one, in fact the model M.52 reached mach 1.6, or 1,000 mph. Considering
it was to have its own undercarriage, take off under its own power like a normal plane, and use the worlds first afterburning jet engine as opposed to
the X-1's rocket, its even more of a shame as it would have (most likely) beat the X-1 into a cocked hat.
Still, 'coulda, woulda, shoulda' isn't quite the same as 'did'.
I can add the Supermarine 545, a supersonic fighter in the F-100/MiG 19 class which was complete and ready for flight when axed in 1955 and the Hawker
P.1121, a mach 2 multirole fighter of similar size and weight to the F-105 which was also canned with the prototype virtually complete two years later
in 1957.
The L133 doesn't count as it was never built but at least the US seems to actually fly the ones it does build.
[edit on 26-9-2006 by waynos]