This is a very interesting article.

From Fox News
The U.S. Air Force nearly detonated an atomic bomb over North
Carolina in 1961 that would have been 260 times more powerful
than the device that destroyed Hiroshima, according to a
declassified report published Friday in The Guardian.

The 1969 document, obtained by investigative journalist Eric
Schlosser under the Freedom of Information Act, details the Jan.
23, 1961, B-52 crash near Goldsboro, North Carolina, that saw
two Mark 39 hydrogen bombs break up in mid-air.

The report said that one of the two bombs behaved exactly as
nuclear weapon is designed to function in wartime and that only
a single low-voltage switch prevented detonation. Fallout could
have been deposited over Washington, Baltimore, Philadelphia
and New York City, according to the report.