Braniff Inc., losing millions of dollars in its struggle to return from bankruptcy, has announced plans to slash service and lay off hundreds of employes and sell two-thirds of its planes.

"We tried, we all tried," Braniff's top managers said in a letter read to employes at a meeting Wednesday night. Braniff's president resigned Wednesday to join a small California airline.

Braniff, which filed for bankruptcy 2 1/2 years ago, reemerged nearly eight months ago under the new ownership of the Pritzker family, which also owns the Hyatt Hotel chain. But the reconstituted airline faced tough competition at its Dallas "hub" from Delta and American airlines, and it has been unable to build enough business to even come close to operating at a profit. The airline has lost $80 million since it went back into operation.

After six months of operations, the airline scrapped its original plan to offer only high-quality, high-priced service, but even deep cuts in fares several weeks ago failed to increase traffic on the carrier. It also laid off 600 of its 2,600 employes at that time in an effort to cut costs. Attempts to find a merger partner also failed.

The letter to employes, written by Braniff Chairman Jay Pritzker and Vice Chairman -- and newly appointed President -- Patrick Foley, said the airline would try to continue operating at a much smaller size.

"Our immediate priority is to continue the airline because we all believe that 'where there's life, there's hope,' and we still have hope that this new plan will permit us the time to grow once again and enable us to prosper in the long term," the letter said.

Braniff officials did not return phone calls yesterday.

Braniff will try to continue with limited service to about 10 cities, including Washington, and about 10 airplanes. The airline is attempting to lease or sell its other planes to other carriers, and is also planning to sell nine of its 12 gate positions at Dallas-Ft. Worth International Airport to American Airlines for $20.5 million in an effort to raise cash to continue operating.

But it remains to be seen how much longer Braniff can continue operating. Airlines industry analysts say that once an airline shows strong signs of serious financial problems, passengers and travel agents tend to shy away from the carrier, fearing that passengers will be stranded if the airline folds and tickets will become worthless or difficult to redeem -- an attitude that compounds the company's financial problems. Similar fears are blamed for hastening Braniff's failure in May 1982.

Braniff said yesterday that it had made arrangements with American Airlines that would allow holders of Braniff tickets to travel on American without having to pay additional fares.

The cities to which Braniff will suspend service Nov. 5 include Austin, Detroit, Houston, Kansas City, Miami, Newark, New Orleans, Oklahoma City, Philadelphia and San Antonio. It will continue flying to Boston, Chicago, Denver, Las Vegas, Los Angeles, Phoenix, New York, and San Francisco, in addition to Washington and Dallas.

Braniff collapsed under the weight of $1 billion in debt in 1982, the first major American airline ever to file for bankruptcy protection. But the Pritzkers, who specialize in turning around failed companies, invested $70 million to take over the airline, returning Braniff to the air last March 1. Analysts say tax credits from the old airline may leave the Pritzkers with an overall profit from the experience.

Many of the employes of the new Braniff were veterans of the old airline. The Associated Press quoted one mechanic at Wednesday night's meeting who is a seven-year veteran of the carrier, as saying, "You feel like you have been hung with the same rope twice."