Kind of reminds me of the little D-21 dronelets that could be carried on either a B-52
https://upload.wikimedia.org/wikipedia/commons/7/79/B-52_with_two_D-21s.jpg
or SR-71
OK, sir, would you like your seat in front of the missile section or behind?
To bad Howard Hughes isn’t around anymore.
Doesn’t sound like a practical solution, but what do I know. Let’s hope the idea was abandoned for impracticality and not for putting more megabucks in the pockets of the military-industrial complex.
1. 747s are civilian aircraft without the redundancy and strength requirements of military aircraft. Loading one up with nukes and waiting for one to inevitably crash is incredibly stupid. How many 747s have already crashed into mountains/piled into the sea so far?
2.When one 747 is used for carrying nuclear weapons ALL 747s become potential targets by enemy forces. How would you like to be aboard a civilian airliner when the "balloon goes up"?
The idea has some merit. If not exclusively for military, but also for commercial uses. A tremendous amount of energy is spent getting the middle off the ground and to 40,000 ft.
Lift off energy would remain constant, however there would be a 40,000+ foot advantage in reduced rocket fuel and weight for each rocket deployment.
Maybe yes. Google Operation Chrome Dome and add missiles to the mix.
No thanks.
https://www.youtube.com/watch?v=ieYsxEe8pkQ 
I have no doubt that when the big one comes any civilian aircraft in the US will be subject to potential use by the military.