My understanding was that Douglas originally selected an aerofoil section for the DC-8 which was too draggy and had to do some leading edge modifications to restore projected performance (the Supersonic dive may have been a planned publicity move related to the mods?) There were more mods to the wings and engine pods/pylons in the -62/-63 models
Early in the 707s career Pan Am had one go into a dive over the Atlantic which may have passed Mach 1 before recovery around 6,000 ft
DC-8 youtubes