Designed with a slide-rule with Pencil and paper. Amazing design and an amazing aircraft.
I was honored to be a member of the Programming team that supported the SR-71 mission for 4 years in the late 80's.
My understanding is that Kelly Johnson and his team went from paper to flying in 18 months. I got to see one of these refueled from a tanker plane. We were on the Iowa-IL state line when the Bird called in, telling us he was slowing down for link-up...he just passed Billings, Montana!