60-6940 accrued 80 flights and 123.9 hours of flying time.
60-6940 was outfitted for drone operations.
940 was one of 2 airframes built from the ground up as M-21’s, to be launch vehicles for the D-21 drone. The designations M-21 and D-21 come from reversing the numbers in A-12, and using M for “mothership” and D for “daughtership.” When ferrying the D-21, the combination was called an M/D-21. The D-21 was powered by a Marquardt ramjet engine that had been developed and tested on the unmanned X-7 vehicle. Ramjets are essentially hollow tubes that use the compression of extremely fast air rushing into the inlet instead of a normal compressor/fan combination like a turbojet. As a result, the ramjet engine is useless below Mach 1.25.
940 carried several D-21s during test flights, but was never used for any launches; instead, she was used as the chase plane for all launches from the other M/D-21, 941.
The only surviving M-21, 940 is on display (as an M/D-21 with D-21 #510 mounted) at Boeing Field’s Museum of Flight near Seattle, Washington. She is one of only 2 blackbirds (along with 927) on display anywhere in the black-and-bare-titanium paint scheme.