The aircraft, from the 60th Air Mobility Wing at Travis Air Force Base, California, made an emergency landing at 11:20 a.m. Pacific Daylight Time at Mountain Home Air Force Base, Idaho, according to a base press release posted on Facebook.
No injuries were reported on the aircraft or the ground, and all crew members were deemed safe and accounted for, the release said.
"The KC-10 was conducting training in support of Mountain Home AFB's Gunfighter Flag Exercise when the refueling boom separated from the aircraft near the range complex," it said.
Photos of the boom showed it landed in a hay field near the base, according to a popular social media page for airmen.
The incident is under investigation. A spokesperson from Travis did not provide additional details.
The KC-10, originally manufactured by McDonnell Douglas as the DC-10 before being upgraded by Boeing, has a single boom, or hose, and drogue centerline refueling system, according to the Air Force. The boom operator sits in the rear of the aircraft to control the boom "through a digital, fly-by wire system," and observe the receiving aircraft.
The aircraft, which carries more than 356,000 pounds of fuel in its six tanks, can transfer fuel at a maximum rate of 1,100 gallons per minute.
Editor's Note: This story has been updated to correct the manufacturer.