A biplane called the Kettering Aerial Torpedo was one of the world's first unmanned flying weapons. Launched in 1918 from a dolly, it was designed to fly for a fixed length of time, then switch off its engine and jettison its wings, dropping a load of 180 pounds of explosives on its target.

Today, much more sophisticated pilotless drones are flying over Afghanistan, including two RQ-1 Predator aircraft equipped with Hellfire antitank missiles.

Drones have two main advantages over manned aircraft: they are cheaper and they can freely operate in dangerous locations.

The Predator, which resembles an upside-down flying spoon pushed along by a propeller at its tail, was widely used in Kosovo in 1999 to monitor the movements of Serbian troops. It is equipped with radar and several cameras. The drone, with a wingspan of more than 48 feet, can stay aloft for more than a day, traveling 450 miles to its target. Flight controllers can be hundreds or thousands of miles away, linked to the drone by satellite.

In February, the Air Force successfully tested the firing of Hellfire antitank missiles that had been added beneath the wings of a Predator.

The Predators are fairly slow and vulnerable, with a maximum speed of 140 miles an hour and a maximum altitude of 25,000 feet. Nineteen of the 60 Predators have crashed; as many as seven may have been shot down over Kosovo and Iraq. The others crashed because of mechanical failures, weather or controller error.

The Air Force is preparing the deployment of a newer drone, the RQ-4 Global Hawk.

With a wingspan of 116 feet, wider than a Boeing 737, the jet-powered Global Hawk can fly at speeds of more than 400 miles an hour and at altitudes up to 65,000 feet. Also, Boeing is building prototypes of the X-45 Unmanned Combat Air Vehicle, which may be able to replace many fighter aircraft.