WASHINGTON (Reuters) -- The most powerful conventional bomb in the U.S. arsenal exploded in a huge, fiery cloud on a Florida test range on Friday after being dropped by an Air Force cargo plane in the last developmental step for the nearly 11-ton "mother of all bombs."

An MC-130E Combat Talon I dropped the 21,700-pound (9,800-kg) satellite-guided GBU-43/B Massive Ordnance Air Blast Bomb, or MOAB, over the test range at Eglin Air Force Base in northwestern Florida, said base spokesman Jake Swinson.

A plume of smoke rose more than 10,000 feet (3,048 meters) in the air and was visible 40 miles (64km) away in Pensacola, Florida. "It looked like a big mushroom cloud filled with flames as it grew and grew and grew," Swinson said after the afternoon test. "It was one of the most awesome spectacles I've seen."

The Air Force called the test successful, saying the bomb separated cleanly from the aircraft with the help of a parachute at 20,500 feet (6,248 meters), glided 41 seconds to its target area and detonated as planned.

Officials said the bomb was developed in only nine weeks to be available for use this spring in the Iraq war, but commanders opted not to use it. Its only previous live test came on March 11, the week before the U.S.-led invasion.

The MOAB, the most powerful non-nuclear U.S. bomb, carries 18,700 pounds (8,482kg) of high explosives, detonating just above the ground when the tip of the 30-foot-long (9.1-meter-long) bomb hits the earth, Swinson said.

Swinson said the bomb was now available to U.S. commanders, but said there were no immediate plans for it to go into production.

The United States has had larger conventional bombs in the past but none in the current U.S. arsenal is as big.