General Dynamics' air-launched cruise missile completed its second flight test yesterday in the Pentagon's billion-dollar flyoff compeitition between that company and Boeing.

Although the Pentagon's cruise missile office declined to characterize the missile's performance on grounds it would prejudice the competition, General Dynamics called yesterday's flight test over the Dugway Proving Grounds in Utah successful.

General Dynamics' Tomahawk missile, designed to be fired from submarines but adaptable to the airlaunched role, was carried aloft by a B52G bomber and fired from a special rack in the plane's bomb bay.

The Boeing entry in the competition has not flown since the flyoff started on July 17.A defense official conceded yesterday that the planned Boeing tests had fallen behind schedule but declined to explain why.

President Carter has opted for building an arsenal of cruise missiles, which are essentially drone airplanes carrying a nuclear bomb, instead of the Air Force's B1 bomber.The B52s will carry the cruise missiles initially, with other planes designed specifically for that role expected to be built in the 1980s.

Pentagon research director William Perry has said that the Defense Department will buy about 3,000 airlaunched cruise missiles costing about $1 million each -- $3 billion total. Counting research costs, the total cost of building air-launched cruise missiles i expected to be $4.4 billion by the Pentagon's estimate.

The winning contractor, Perry said, "is certainly looking at a production contract well in excess of a billion dollars and probably in excess of $2 billion."

However, the loser of the flyoff between General Dynamics and Boeing also is expected to build some of the 3,000 cruise missiles because the Pentagon has made the policy decision to have two sources for the weapons.

The Pentagon expects to select the winner of the competition next year. The flight tests will be conducted throughout 1979 under present planning.