That was the M-21/D-21 project. After a failed release and a crash the D-21 drones were fitted with solid boosters and released from a B-52.
https://www.sr-71.org/blackbird/m-21.php
https://en.wikipedia.org/wiki/Lockheed_D-21
Only four operational missions were flown, all over China with the last one disappearing somewhere over Yunnan. Perhaps they found it...
(just kidding, they did).