The Air Force said yesterday that it is zeroing in on why the air bag on the escape capsule of the B1 bomber that crashed Wednesday failed to protect the three-man crew from death and injury.

Air Force generals also told a news conference at Edwards Air Force Base in California near the crash site that the bomber-launched cruise missile program will be delayed because the lost B1 was the only one of four prototypes equipped to test cruise weapons.

The bomber plummeted into California's Mojave Desert after its pilot lost control while testing how slowly it could fly at low altitude with its wings swung back along the fuselage in the minimum lift position, Air Force sources said.

The plane stalled despite the crew's efforts to regain control by increasing power and trying to push the wings forward toward the high-lift position, informed sources said.

A crew member squeezed the ejection handle when the pilot failed to regain control, triggering rockets that separated the crew compartment, or capsule, from the bomber plunging toward the desert, the sources said. The capsule's three parachutes deployed, slowing its descent, but the capsule hit the ground with such force that one man was killed, another's back was crushed and the third suffered a collapsed lung.

During descent, the capsule is supposed to swing around from a nose-down position to become horizontal. But the crew may have ejected at such low altitude, officials said, that the capsule did not have time to swing around.

The air bag designed to inflate on the bottom of the capsule when it is horizontal would have done little to absorb the impact if the capsule landed in the nose-down, vertical position, the sources said.

"I have people looking at this right at this moment," Maj. Gen. Gordon E. Williams, head of the Air Force accident investigations board, told reporters at Edwards when asked why the air bag failed to cushion impact. "I can't answer that now."

Air Force leaders there refused to describe what happened just before the plane went out of control or theorize about why it crashed. They said they are waiting until the Air Force investigation is complete.

Maj. Gen. Peter W. Odgers, commander of the Flight Test Center at Edwards, said the B1 was at 3,500 feet and flying at about 180 knots when it went out of control. He said it was being set up for a test of how the four-engine bomber would perform with one engine turned off but never reached the starting point for that maneuver.

Odgers did not disclose what maneuver was being conducted when the $200 million bomber went out of control. Other informed Air Force sources said the fateful maneuver involved putting the plane through low-speed, low-altitude and low-lift tests to establish performance limits with its wings swept back in the high-speed position.

The nation's most experienced B1 pilot, Doug Benefield, 55, chief test pilot for Rockwell International Corp., builder of the bomber, was sitting in the right seat when the plane went out of control.

An Air Force test pilot, Maj. Richard V. Reynolds, 35, was in the left seat, apparently flying the plane, and Capt. Otto Waniczek Jr., 30, was behind them in the flight-engineer space.

Asked why the experienced Benefield would not have grabbed the controls from Reynolds when the plane went out of control, an Air Force general who has flown the B1 said, "At that point, there probably was nothing to grab" -- meaning the bomber had stalled and did not respond to the flight controls.

Benefield died from internal injuries suffered when the capsule slammed into the desert, Reynolds' back was crushed and Waniczek incurred a collapsed lung from a blow to his chest. The two hospitalized fliers are recovering.

The lost B1 was one of two test models being flown to identify problems with the bomber that President Reagan put into production in 1981 after President Carter canceled it in 1977. The crashed B1 was to test the launching mechanism for Air Force cruise missiles.

The Air Force said another B1 will have to be equipped to test the internal and external cruise missile launchers, causing delays of undetermined length.

The first production model of an upgraded B1, the B1B, is to be rolled out in an elaborate ceremony at Palmdale, Calif., Tuesday. The bombers are expected to be flying with Air Force squadrons next year.

Reagan has said he will buy 100 B1Bs for no more than $20.5 billion in fiscal 1981 dollars. Skeptics have said they expect Rockwell, its subcontractors and political backers to launch a mighty lobbying effort to keep the B1B in production beyond that total.