United Airlines yesterday proposed the most wide-ranging fare discounts yet in the continuing and escalating domestic air travel price war.

The nation's largest airline said it would ask the Civil Aeronautics Board today for permission to extend Super Saver discount fares to all United routes over 900 miles. The plan would expand the discount fares, with savings of 30 to 40 per cent off regular coach fares, to each of the 110 coties United serves.

From Washington, the new low fares would be available to 34 different cities.

Until now, United offered Super Saver fares between a total of 26 pairs of cities, all of which were major transcontinental routes. With CAB approval, the discount fares would be offered between a total of 1,500 pairs of cities on United's extensive route system.

Under United's proposal, the new fares - withdiscounts of about 40 percent Monday through Thursday and 30 percent Friday through Sunday - would be available for travel beginning March 18. An exception would be available beginning April 26.

Also on April 26, the new fares would also replace existing Super Saver cross-country fares which now offer 30, 40 and 45 percent discounts depending on the day of the week.

All the new faries, which would be available on a maximum of 36 percent of the seats on all flights, would remain in effect through May 31. They would apply on round-trip travel only to trips between seven and 45 days in length, not counting the day of departure. Reservations must be confirmed at least 30 days before departure.

Howard Putnam, United group vice president for marketing, said United hopes the fares would stimulate "soft" spring months, especially on longer routes that frequently have empty seats this time of the year.

The discount fares have proven most popular on those routes, he said. He noted that United's Freedom fares, which feature discounts of up to 20 per cent, will continue to be available on routes shorter than 900 miles.

Putnam said results of the spring discount fare expansion would help the airline decide what to do about summer fares.

Alluding to the fact that United officials were skeptical at first about the ability of the discount fares to generate new passengers, Putnam said, "We now believe Super Saver fares at the discounts we're proposing are capable of generating sustained profit for the airlines. We have experienced substantial growth in markets currently offering the fares."

In a related development, the CAB yesterday approved Braniff Airways' proposal to institute a 50 per cent fare reduction "Home Free" plan on certain routes afte Braniff agreed to modify certain cancellation and no-transfer conditions.

Braniff resubmitted a proposal that would allow a traveler with a discount ticket who wants to cancel a reservation to transfer the ticket to another within 14 days of the flight or to forfeit half of the ticket price. Braniff's original proposal would have allowed no transfers. A person cancelling a reservation would have had to forfeit the entire ticket price under that plan.