An Air Force B-52 bomber flew across the central United States last week with six cruise missiles armed with nuclear warheads that were mistakenly attached to the airplane's wing, defense officials said yesterday. The Stratofortress bomber, based at Minot Air Force Base in North Dakota, was transporting a dozen Advanced Cruise Missiles to Barksdale Air Force Base in Louisiana on Aug. 30. But crews inadvertently loaded half of them with nuclear warheads attached.