The B1 bomber took its first test flight seven years ago, but reappraisals of its merits have left it little more than an aging stopgap, certain to be overtaken by new methods of war and new Soviet defenses.

The B1 is a major improvement over the B52, whose pilots are often younger than their aircraft.

When U.S. strategists discovered in the late 1950s that the Soviets could shoot down high-flying aircraft such as the famous U2 spy plane downed in 1960, they trained B52 pilots to fly the bombers only a few hundred feet above the ground.

That helped frustrate detection by Soviet radar and foil antiaircraft missiles, but strained the B52, which was designed to fly much higher.

The four prototype B1s at Edwards Air Force Base in southern California are designed to fly as low as 200 feet, by one estimate. They give the four crew members a smoother ride and keep them alert and fresh during long rides in dense, turbulent air close to the ground.

To cut costs and increase range, top speed was reduced to slightly above the speed of sound. The B1 is designed to travel 8,000 miles.

The B1 is 151 feet long, with a 137-foot wingspan that becomes 78 feet as its wings sweep back at top speed. With its reduced size and new "Stealth" technology that fools enemy radar, the B1 is more difficult to detect on radar than the B52.