A Pentagon special study group is preparing to recommend that President Reagan build an updated version of the B1 as the U.S. bomber of the future, defense sources said yesterday.

The plan, to be sent soon to Defense Secretary Caspar W. Weinberger, calls for earmarking between $15 billion and $20 billion to build about 110 B1s, with the first squadron of 15 to be ready for duty by late 1986.

The new B1, according to its advocates in the study group, would look like the bomber President Carter canceled but would actually be vastly improved, incorporating some Stealth techology for eluding Soviet radar coverage.

Advocates intend to cite Stealth tricks to rebut Carter's contention that Soviet defenses would make the B1 an obsolete way to fire nuclear weapons at Russia, but they also will stress that the new bomber could save Persian Gulf oil fields in a conventional war.

The argument here is that some of the B1 bombers would be armed expressly for a Persian Gulf role and be kept ready to fly there before an assault on oil fields could be completed.

"Smart" weapons with TNT-type warheads -- some of them guided by a laser beam for pinpoint accuracy -- would be put on the Persian Gulf ready force of bombers to stop an armored column, for example. Advocates contend this would buy time, enabling the president to send in other forces before the conflict was decided.

Although it would be easier to launch such bombers from bases in Egypt or Israel to the Persian Gulf or Africa, Pentagon specialists in the bomber study group warn that foreign governments may not go along with this during complicated crises likely to erupt in the 1980s and the 1990s. Therefore, they argue, the president should be given the option to send bombers in from the United States in a hurry.

Rivals to the updated B1 during the secret Pentagon study, which started in earnest last August, have been an elongated version of the F111 and a brand-new Stealth aircraft, a plane incorporating all the latest features for being virtually invisible to enemy radar.

The F111 lost out early in the study, sources said, because it would not be worth the cost. Taking F111 fighters away from the Tactical Air Command to convert into bombers also would create a whole crop of problems in that outfit, study group insiders added.

Although a Stealth bomber still looks promising, the study group is reluctant to put all the chips on this option before lingering questions on technology and cost are answered.The prevailing view, sources said, is to go ahead with the updated B1 and buy Stealth bombers as soon as those questions are answered. This could well involve buying two bombers at once, although the hope of the study group is to phase the production so the Stealth begins near the end of the B1 run.

The B1 advocates on the Pentagon study group have marshaled data showing that relying on the existing B52 bombers through the year 2000 would be risky, with metal "creep" around rivets one of the perils being cited. They contend the better idea would be to start immediately on replacing the B52 fleet so the enormous cost could be spread out over the next 20 years, thus helping to preserve congressional support.

In fact, shortly after Reagan became president, some of the members of the Pentagon bomber group recommended adding $1.7 billion to Carter's fiscal 1981 defense budget to accelerate the new bomber program. Weinberger decided this was too much too soon and settled for adding $2.4 billion to the fiscal 1982 Carter budget for the new bomber.

If Weinberger buys the package being prepared for him, 350 B1 and Stealth bombers would be set down as the objective between now and the end of the century.

Although many of the planned improvements for the B1 that Carter canceled in favor of cruise missiles in 1977 are highly secret, two are known: putting radar-absorbing material on the leading edges of the wings and changing the air inlets on its engines so radar does not reflect from their innards.

Also, to lower the cost and minimize technical problems of the new B1 virtually certain to be recommended to Weinberger, the bomber's wing would sweep back and forth in a shorter arc than the old one.

Backers of building an updated B1, which would be produced by Rockwell International, believe politics are on their side. They said that Reagan's best, possibly his only, chance to make a big imprint on the nation's strategic forces is to build the new bomber.