“It took 27 months to go from a paper napkin to an operation craft called the SR-71.”
Kelly Johnson was still alive and in charge then!!!!
The SR-71 was not built on a production line. Neither was the first P-51 which was a flying prototype. The F-35 is in pre-production phase. This is where they iron out the kinks (hopefully) while they arrive at the first production bloc configuration. There is a lot of plant layout work being done simultaneously as production ramps up.
The program is struggling, no doubt.
And they had an A-12 Oxcart to start from.