It was proposed in 1982, with PW2000s, and was rejected because all the B-52s were to be retired in the 1990s. Rolls tried in the mid-90s, with RB211-535s, and it was turned down because the AF did its sums wrong. (They forgot that gas from a KC-135 costs a bit more than gas at the pump.) Now it's back because there are efficient engines the right size for an eight-engine solution, which makes the engine-out case easier.