CFE said:
Did the XB-70 + X-15 combination ever make it as far as wind tunnel testing, or was the painting of the X-15 launching from the XB-70 merely a piece of PR artwork?
It was more PR than engineering. By that point, it was well known that launching something off the back of a Mach 3 plane was suicide.
There were also plans for launching from underneath a B-70 (such as the orbital X-15B proposal), but those would have required either landing gear modifications or launching from a trolley of some kind.