The B-52 "Stratofortress" lost one of its eight turbofan engines when it was around 28 miles from the Minot Air Force base in North Dakota, a spokesman said.
The aircraft, which is designed to hold up to 70,000 pounds (32,000 kg) of nuclear warheads, was not carrying any weapons.
It landed safely using the remaining seven engines .
The eighth engine landed in an unpopulated area and no injuries were caused, the spokesman said.
He added that the crew "declared an in-flight emergency when the pilot discovered that an engine departed the aircraft."
He said: "There were no weapons on board and it was a local training mission. The aircraft landed safely with no injuries to the five personnel on board.”
Six crew members were killed and investigators said the probable cause was a system malfunction.