Steven P. McNicoll wrote:


The X-32 and X-35 were properly numbered as they were technology
demonstrators, not prototypes.


They were technology demonstrators, which were expected to be
developed directly into fighter prototypes. Therefore, the X-32/35
could have been properly designated as XF-24/25. Assuming that #25
would have been the winner, the JSF _prototypes_ would have become the
YF-25.

[prefix X = Experimental, Y = Prototype; it used to be different (X =
prototype, Y = service test), but this was changed decades ago]

Andreas