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By Will Dunham

WASHINGTON, Nov 21 (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-tonne "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 (64 km) 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 nonnuclear U.S. bomb, carries 18,700 pounds (8,482 kg) of high explosives, detonating just above the ground when the tip of the 30-foot-long (9.1-metre-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.

The MOAB is envisioned as a successor to BLU-82, the 15,000-pound (6,800-kg) "Daisy Cutter."

The "Daisy Cutter" was used to clear helicopter landing areas in the Vietnam War and was used in the 1991 Gulf War and in 2001 in Afghanistan. In the latter two conflicts, U.S. commanders used the "Daisy Cutter" partly for the psychological effect of such a massive blast.

Swinson said it was the last of four developmental tests for the MOAB -- nicknamed the "mother of all bombs" by some in the military. The two live tests were preceded by two inert tests.

Lynda Rutledge, MOAB program manager at Eglin, said there were minor modifications to the MOAB tested on Friday compared to the one detonated in March, adding that the latest test sought to give commanders a chance to understand how the big bomb performs, particularly relating to targeting.

Poor weather forced a postponement of the test on Tuesday and a problem with a laptop computer aboard the plane carrying the bomb forced another delay on Thursday, officials said.