I talked to a McDonnell Aircraft structural engineer in the 80s. He said something like Old Man McDonnel told us to design for a 0.85 factor of safety on the F1

I talked to a McDonnell Aircraft structural engineer in the 80s. He said something like Old Man McDonnel told us to design for a 0.85 factor of safety on the F101. If anything broke during testing, we'd redesign it. I'm not sure if that was how it really worked out.