Here's a short video of the incident.
Supposedly, some D-21s are Google Earth visible in the boneyard in Arizona. Dunno if they ever flew any operational spy missions as designed.
The on board self defense fighter probably made sense at one time when bombers had enormous range and needed a way to fend off enemy interceptors once near the target. Hard to see the little XF-85 taking on a Mig, but proof of concept is just that. Think it was tried with the B-36 with a larger fighter.
Still a twitchy deal flying near a large aircraft turbulence. heck, ATC even warns of flying near where a large plane was some time before, let alone flying underneath the damn thing!
I’m sure parasite uav’s launched from space will be the next thing. It would likely be successful since they can land on the ground and be reused. No need for docking in space.