Which B-47s had to perform a half loop to drop nuclear bombs?

The B-47 Stratojet's early mission profiles included loft bombing of nuclear weapons. The tactic was made public on May 7, 1957, at Eglin Air Force Base: a B-47 would enter its bomb run at low altitude, pull up sharply (3.5 g) into a half loop, release the bomb under automatic control at a predetermined point in the climb, and then execute a half roll, completing a maneuver similar to an Immelmann turn or a Cuban half-eight. The bomb continued climbing for a time in a very high arc before falling on a target located a considerable distance from the release point, while the maneuver allowed the bomber to reverse direction and fly away from the target. Air Force pilots nicknamed the procedure the "LABS maneuver," for Low-Altitude Bombing System. Also called toss bombing, it allowed bomb runs that did not fly directly over the target. The B-47 kept pulling up until reaching a higher altitude before rolling upright.

The Stratojet could fly nearly twice as fast as the B-29 Superfortress and dropped megaton-class weapons capable of producing fireballs up to 40,000 ft high and a blast radius of hundreds of miles. The maneuver was only possible because of the B-47's design, but it left cracks in the aircraft's structure, which could not withstand repeated stress on the fuselage. Training for this type of attack imposed high, repeated loads, so the airframe's service life would have been severely limited by metal fatigue, and the maneuver was dropped.