Douglas Aircraft Co. will lay off 1,200 workers at its Long Beach and Torrance facilities this year and hopes to reduce its work force by another 1,500 employees in 1991, a company executive said.

Layoff notices will be distributed this week to supervisors, salaried workers and some unionized hourly employees, said Joe Pirkle, chief financial officer. The ranks of aircraft assembly workers will not be reduced, he said Monday.The layoffs will take effect in December, Pirkle said.

The aerospace firm plans to reduce its work force by another 1,500 employees next year through attrition, he said.

Beset by slumping defense spending, Douglas already had laid off 5,000 workers and 2,200 contract employees since May.

The layoffs are part of a $700 million cost-cutting program ordered by Douglas' parent company, McDonnell Douglas Corp. of St. Louis.