Now the Pentagon is trying to make invisible what it said publicly about its invisible bomber.

Richard D. DeLauer, Pentagon research director, blurted out during a public hearing of a Senate Armed Services subcommittee Feb. 24 that the first Stealth bombers, designed to be virtually invisible to enemy radar, will be flying in 1991.

This is only about five years after the arrival of the much less advanced B1 bomber, which will cost either $200 million or $400 million apiece, depending on the Air Force estimate or the higher Congressional Budget Office projection. Congressional critics failed in several attempts last year to force the Pentagon to forgo the order for 200 B1s and use the money for Stealth.

Rather than let the 1991 date for Stealth stay public, Pentagon officials confirmed last night that they have asked the Senate Armed Services Committee to consider that part of DeLauer's testimony secret and not for publication in hearings later this year.

Asked to explain classifying information after its release in open session by a Pentagon executive, Defense Department spokesman Henry E. Catto Jr. said: "I'm not going to have anything to say about it."

For the moment, the committee has inked over the 1991 date in the hearing transcript available to the public. This was the exchange between Sen. Carl Levin (D-Mich.) and DeLauer in the strategic and theater nuclear forces subcommittee open hearing:

Levin: Gen. Richard Ellis former commander of the Strategic Air Command told us last year that he did not see major design problems to put the Stealth effort in the high-risk category. Do you agree . . . ?

DeLauer: No, I disagreed . . . then and, now that we are further into it, I would disagree even a little bit more, but for a different reason which I can get into in classified session.

Levin: You disagreed with him on that assessment?

DeLauer: Yes.

Levin: Has the IOC initial operational capability for the Stealth bomber slipped?

DeLauer: No.

Levin: What is that IOC?

DeLauer: IOC now is 1991. It has not slipped.

Some trade publications have published the 1991 date, although the Pentagon is still trying to classify it.

The Pentagon also classified parts of another public hearing by a Senate Armed Services subcommittee on manpower Feb. 26. Deleted were some exchanges between Sen. Sam Nunn (D-Ga.) and military witnesses on how many forces the Joint Chiefs of Staff said they would need to carry out national security policies.