Blue Danube was the first operational British nuclear weapon. It also went by a variety of other names, including Smallboy, the Mk.1 Atom Bomb, Special Bomb and OR.1001, a reference to the Operational Requirement it was built to fill.
The RAF V bomber force was initially meant to use Blue Danube as their primary armament at a time when the first hydrogen bomb had not been detonated, and the British military planners still believed that an atomic war could be fought and won using atomic bombs of similar yield to the Hiroshima bomb. For that reason the stockpile planned was for up to 800 bombs with yields of 10-12 kilotons. V-bomber bomb bays were sized to carry Blue Danube, the smallest-size nuclear bomb that was possible to be designed given the technology of the day (1947) when their plans were formulated.