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Because the US needed a new strategic, long range bomber.
in us air force termonology, "stategic" basically refers to our bomber aircraft.
The B-52 is not a fighter, it is a heavy strategic bomber.
More than likely, the B-52 Stratofortress, the B-52 is a huge strategic bomber big enough to carry the atomic bomb.
Strategic bomber, submarine ICBM, ground ICBM