The nation's "Stealth" bomber of the future is shaping up as a radically advanced "flying wing" designed to look like nothing more than a speck, if that, on a Russian radar screen, informed sources said yesterday.

Enthusiasts claim that the building materials of today, such as paper-thin sheets of graphite fibers, will overcome the problems that caused the flying wings of the 1940s to fail.

The modern composites can be bent and curved to slip through radar beams rather than bounce them back to enemy gunners, they claim. Even the Stealth engine would be slipped inside this new flying wing to avoid presenting a sharp surface.

The glittering promise some lawmakers see in Stealth is prompting them to try and convince the Pentagon to leapfrog Stealth over the B1 bomber, which President Reagan intends to build first. At least one amendment to force this course on Reagan will be offered this week as the Senate considers the Pentagon's fiscal 1983 procurement bill.

Air Force leaders want to stick to the two-bomber approach, with the Rockwell International B1 entering service in 1986 and Northrop Stealth in 1991. But some of them think the new pressure to cut the defense budget, together with extravagant claims for Stealth, puts their prized B1 bomber in new jeopardy.

Aside from the political challenges ahead on the Senate and House floors, B1 advocates in the Air Force warn that lots of technical questions remain about a flying-wing Stealth before the nation grabs for that bird in the bush rather than the one in hand.

Skeptics of the flying wing argue, for example, that building a bomber without a tail to help control it invites the same stability problems that plagued the aluminum flying wings tried in the 1940s, sometimes with disastrous results.

And if Stealth does run into problems during development, as some Air Force leaders predict, there is a fear that money to solve them will come from the B1 account even if that less-advanced bomber is already in production. Reagan intends to buy 100 B1s for $28 billion.

Some Air Force leaders complain privately that Northrop is jeopardizing the B1 by making extravagant claims about what a flying-wing Stealth could do, when it could be ready and how much it would cost.

Whether that is fair criticism or not, Air Force officials were so upset about Stealth descriptions in an April 19 Business Week article on Northrop that they have launched an investigation of possible security violations, according to Pentagon sources.

The Air Force, when queried by The Washington Post, refused to confirm or deny that it had begun such a probe.

Business Week asserted that Northrop's game plan is to substitute Stealth for the B1, writing: "Having profited by the B1's birth, Northrop now clearly hopes to shoot down the Rockwell plane . . . . Northrop's speed in flying its Stealth is critical to this debate. Insiders say the company aims for test flights in late 1984, vs. a March, 1985, target for the B1B."

The new interest in reviving the flying wing was underscored at the National Aeronautics and Space Administration's Research Center at Langley, Va., last week when an executive there refused to allow a reporter to look at an unclassified model of a delta-winged plane in the wind tunnel because the official expected the Pentagon to cloak it in secrecy soon.

Military and commercial planes already built have demonstrated that such material as paper-thin sheets of glistening graphite can be pasted together to make wings of unprecedented strength and lightness. The Marines' AV8B Harrier being built by McDonnell Douglas, for example, has such wings.

Just as the advent of fiberglass enabled boat builders to bend hulls into shapes that wood could not endure, so it is with composites available to the Stealth. The new bomber, sources said, will be molded into soft curves that will not bounce radar beams to enemy gunners watching radar scopes.

Northrop lit up the skies of military aviation in 1947 by flying its YB49 Flying Wing bomber, powered by eight General Electric jet engines. GE also builds the engines for Stealth.

The YB49 set a record for jet aircraft of the day by remaining aloft 9 1/2 hours. The second YB49 produced crashed on June 5, 1948, as the wing broke, killing all five crewmen. Although the Air Force continued to invest in flying wings, none was produced in quantity.

The search for a way to make a bomber invisible to radar, or no more than a speck, together with the new building materials, has given the flying wing a new lease on life. Besides accommodating shapes that evade radar, today's composites can be made into a silky-smooth surface that enables a plane to slip through the air with less drag, thus increasing range.

Also, the elusiveness of Stealth reduces the need to rely on speed to penetrate enemy airspace. Planes that fly slowly burn much less fuel than those zooming along at supersonic speeds of more than 670 miles an hour.