The 1961 Goldsboro B-52 crash was an accident that occurred in Goldsboro, North Carolina, on January 24, 1961. A B-52 Stratofortress carrying two 3-4 megaton Mark 39 nuclear bombs broke up in mid-air, dropping its nuclear payload in the process.

In 2011, Lt. Jack Revelle, the bomb disposal expert responsible for disarming the device, said “As far as I’m concerned we came damn close to having a Bay of North Carolina. The nuclear explosion would have completely changed the Eastern seaboard if it had gone off.” He also said the size of each bomb was more than 250 times the destructive power of the Hiroshima bomb, large enough to have a 100% kill zone of 23 km. Each bomb would exceed the yield of all munitions (outside of testing) ever detonated in the history of the world by TNT, gunpowder, conventional bombs, and the Hiroshima and Nagasaki blasts combined.

Makes me wonder, why on earth would they fly these things around other than actually using them? And it's not just one, but two of em on the same plane.

Starfish Prime was a high-altitude nuclear test conducted by the United States on July 9, 1962, a joint effort of the Atomic Energy Commission (AEC) and the Defense Atomic Support Agency. The Defense Atomic Support Agency apparently became the Defense Nuclear Agency in 1971.

A Thor rocket carrying a W49 thermonuclear warhead (manufactured by Los Alamos Scientific Laboratory) and a Mk. 2 reentry vehicle was launched from Johnston Island in the Pacific Ocean. The explosion took place 250 miles (400 km) above a point 19 miles (31 km) southwest of Johnston Island.

[...]

Starfish Prime caused an electromagnetic pulse (EMP), which was far larger than expected, so much larger that it drove much of the instrumentation off scale, causing great difficulty in getting accurate measurements. The Starfish Prime electromagnetic pulse also made those effects known to the public by causing electrical damage in Hawaii, about 1,445 kilometres (898 mi) away from the detonation point, knocking out about 300 streetlights, setting off numerous burglar alarms and damaging a telephone company microwave link. The EMP damage to the microwave link shut down telephone calls from Kauai to the other Hawaiian islands.

While some of the energetic beta particles followed the Earth's magnetic field and illuminated the sky, other high-energy electrons became trapped and formed radiation belts around the earth. These man-made radiation belts eventually crippled one-third of all satellites in low Earth orbit. Seven satellites failed over the months following the test, as radiation damaged their solar arrays or electronics, including the first commercial relay communication satellite, Telstar, as well as the United Kingdom's first satellite, Ariel 1.

They would've blamed the Soviet Union for it.

Dozens of Atomic Warheads Lost In Sea by Superpowers, Study Says
By ANDREW ROSENTHAL, Special to The New York Times
Published: June 7, 1989

WASHINGTON, June 6— Accidents involving American and Soviet ships, bombers and rockets have left at least 50 warheads and nine nuclear reactors scattered on the ocean floors since 1956, according to a comprehensive study of naval accidents that was released today.

So far, the United States has acknowledged that radioactive material leaked into the ocean from only a hydrogen bomb dropped accidentally into the ocean south of Japan in 1965 by an aircraft carrier.

...

A bomb that went "broken-arrow" wont explode -cannot explode. Not due a crash...
At least they say so.

In 2013, ReVelle recalled the moment the second bomb's switch was found. “Until my death I will never forget hearing my sergeant say, 'Lieutenant, we found the arm/safe switch.' And I said, 'Great.' He said, 'Not great. It’s on arm.'”