It was based on the TX 21 "Shrimp" prototype that had been detonated during the Castle Bravo test in March 1954.

While most of the Operation Castle tests were intended to evaluate weapons intended for immediate stockpile, or which were already available for use as part of the Emergency Capability Program, Castle Bravo was intended to test a design which would drastically reduce the size and costs of the first generation of air-droppable atomic weapons (the Mk 14, Mk 17 & Mk 24).

Aircraft speed at release was limited to 400 knots (740 km/h), so as to not exceed an opening shock of 6,000 Gs on the parachute harness.

The bomb was equipped with a two-stage deployment system, including a 24 feet (7.3 m) main ribbon canopy which provided up to 108 seconds of retardation.

To carry the Mk 21, the B-47 required the installation of special fin recesses in the bomb bay doors.