i think sometime in the 60's, after retiring their older refueling aircraft for the KC-135.
they developed both methods, but the USAF ended up going all in on the flying boom set up because of the strategic air command's needs. the big bombers tended to be very fuel hungry, plus they were frequently being kept in the air for prolonged periods of time. so they were refueling often. the probe and drogue system couldn't pump fuel fast enough to refuel the big bombers efficiently. so they went in for the booms, which while heavier and needing a bigger plane, could move the fuel fast.