Didn't realise it was the fastest airplane ever built, that sheds some light on it. It is really a greater engineering feat than anything in the space program?

It depends on if you count the X-15 or the Shuttle, but neither of those were air-breathing.

The shuttle should be considered a glider (a lousy one) when landing. The X-15 is a rocket with wings and should be called a spaceplane - it crossed the 100 Km border twice after all, and couldn't take off by itself.

Agreed. The X-15 even had a RCS for when control surfaces no longer had enough air to bite into.

The coolest thing is the analog computer that seamlessly went between control surfaces and RCS as if it were flying using surfaces within an atmosphere. It may not be a "real" plane, but that doesn't make it any less amazing.

I'd love to see the X-15 progress into the X-20 and onwards.