Such a great book that is becoming harder to find a decent copy of these days.

There's a great sketch that Kelly Johnson did of the A12. This was in September of 1958(!) and he had a rough top speed of Mach 3.0, with a possibility of Mach 3.2. And it existed on nothing but a sheet of paper and in his mind. No computers with advanced software and super computers with petaflops of power.

https://www.thesr71blackbird.com/History/CIA/a-12-blackbird-...