A-12 flew faster and higher and was iterated in design details to become the SR-71. Some documentation of the J-79 engine indicates an operational envelope to 4.2M.
Plumb forgot about the A 12. But, it basically became the SR 71 so..
My wife is from around Ishpeming Michigan. At a gathering up there, we got to talk to a cousin of Kelly Johnson.
Told me that Kelly wouldnt say shit if his mouth was full of it. Thats how quiet the man was. Said the man virtually lived airplane design and was always thinking.