The Blackbird SR-71C was a one-off variant formed from two other aircraft.
The SR-71 Blackbird is one of the most unusual aircraft ever built. It stands out for both its unique design and its incredible speed. But there was one variant of the series that was even more unusual. The one-off SR-71C was a training aircraft - pieced together from two halves of other SR-71 aircraft. After initial problems, it flew fine but only saw limited use.