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YF-12: The Super Secret Plane That Smashed Speed Records During its nine years of existence, the YF-12 had slightly less than 300 flights, of a total of about 450 flight hours. And it was thought to be one of the fastest jets ever to fly. Before the famous SR-71, there was the YF-12, which emerged from Lockheed Martin in the 1960s. It had a short life and was never actually used operationally by the military, but it did form the basis for the SR-71, which had a much longer life. Only three of ...