I thought the J47s were needed to get the B-36 off the ground in hot and high, Max AUW conditions.
That, and just as important, to increase the airspeed over the target, to avoid the effects of the nuclear weapon deployed....the get away quickly, scenario.
I clearly remember these aircraft flying overhead in southern California in the mid-1950's.
Even though they were quite high (30,000 feet+) you could clearly hear the airplane...not the engines, the beat from the six 19 foot diameter propellors.
It would rattle windows, big time....no mistaking that unusual sound.