I'm not sure of all of the numbers but the DC-8 was the first airliner to break the sound barrier, in a controlled dive out of FL 500
DC-8 was used to validate Einstein's theory of special relatively. It was flown with an atomic clock and compared to a stationary atomic clock, and the clock in the DC-8 was slower.
Since the reverse thrust doubled as a speed brakes. the DC-8 was chosen to determine handling capabilities of high bypass engines at thrust reverser deployment as part of the investigation of the Lauda disaster.