The D-21 was also launched from from the back of a M-21, which is a variant of the SR-71Lockheed D-21 reconnaissance drones were launched from B-52Hs.
https://theaviationgeekclub.com/tag...h-3-ramjet-powered-reconnaissance-drones/amp/
I guess it does make more sense to drop it from under the wing rather than on the back, it’s a lot simple, and safer.That proved to be dangerous in flight tests. In actual D-21 reconnaissance missions, B-52Hs were used as carrier aircraft.
M-21 was developed from A-12, the precursor to SR-71.