The B28 (originally Mk 28) was a thermonuclear bomb carried by U.S. tactical fighter bombers and bomber aircraft.

The Mk 28 was produced from 1958 through 1966. It used the W28 lightweight, Class D warhead (also shared with the TM-76 Mace surface-to-surface missile and the GAM-77 Hound Dog air-launched cruise missile). After 1968 it was redesignated B28.

20 different versions were offered, distinguished by their yield and safety features. The B28 used the ‘building block’ principle, allowing various combinations of components for different aircraft and roles. The principal configurations were:

• B28EX: streamlined external-carriage version for free-fall delivery; no parachute.
• B28RE: streamlined external-carriage version with parachute retarder
• B28IN: unstreamlined internal-carriage version for free-fall delivery; no parachute.
• B28RI: unstreamlined internal-carriage version with parachute retarder
• B28FI: unstreamlined internal-carriage version with parachute for laydown delivery; used only by SAC B-52s.

The B28 had a diameter of about 22 in (28 cm), with a length varying between 96 in (2.44 m) and 170 in (4.32 m) and weight of 1,700 lb (771 kg) to 2,320 lb (1,053 kg), depending on version. Explosive yield was 1.1 megaton for Mod 1 warheads, 350 kiloton for Mod 2, 70 kiloton for Mod 3, and 1.45 megaton for Mod 5. It could be configured for airburst or groundburst detonation.

4,500 B28s were produced. The last examples were retired in 1991.

The B28 should not be confused with the XB-28 Dragon, a prototype high-altitude version of the World War II-vintage B-25 Mitchell light bomber.