"Designing the plane may only have taken four years, but training the staff, organising the workshops and mastering the production tools took much longer."

Lockheed had no experience with titanium prior to OXCART. 95% of the first 6,000 sheets of titanium had to be scrapped.

It was the first production design to incorporate low-observable shaping.

It was 50% faster than anything Lockheed had previously tried to build, and four times faster than the U-2.

And they literally did all that between April 1958 and April 1962.

So, yes, it was designed from scratch as nothing useful from prior projects read across to it.