In January 1961 a Boeing B-52 Stratofortress carrying two 3–4-megaton nuclear bombs broke up in mid-air near Goldsboro, North Carolina dropping its nuclear payload in the process.

Ejecting at 9,000 ft, five crewmen landed safely. Another ejected, but did not survive the landing, and two died in the crash. 

Information declassified in 2013 showed that one of the bombs came close to detonating, with three of the four required triggering mechanisms having activated.

Source: Wikipedia