United Airlines won tentative approval yesterday for a new non-stop route between Tokyo, Japan and the West Coast cities of Seattle and Portland.

At an open meeting, the Civil Aeronautics Board instructed its staff to draw up a draft order giving United the route on condition that it offer travelers the low fares it proposed during a lengthy hearing process.

United said it would offer a basic economy fare 35 per cent below those currently available between the Pacific Northwest and Japan.

Currently, Northwest Airlines is the only U.S. carrier providing passenger service on the route. Other carriers seeking the route were Braniff International Airways, Continential Airlines, Pan American World Airways and Western Airlines.

The tentative award of the route to United marks a real change in the board's attitude toward the nation's largest carrier. United has complained bitterly that it is always denied new routes because it is the "big boy on the block."

The board's decision reversed an initial ruling by an administrative law judge who said the route would not support a second carrier. The judge also had suggested that a carrier's offer to provide low fares shouldn't be a key factor because the airline couldn't be held to its promise.

After formal approval by the CAB, the decision is subject to review by President Carter, as are all CAB decisions affecting international rates or routes.

In a related development,a CAB administrative law judge recommended to the board that Allegheny Airlines and North Central Airlines be granted nonstop service between Baltimore and Detroit. United currently operates one daily nonstop flight in each direction between the two cities.

In his initial decision, ALJ Arthur S. Present suggested that North Central be directed to provide the service, while Allegheny be allowed to operate the service if it desires. President's decision is subject to review by the board.