Boeing Co. won a major competition yesterday when Delta Air Lines announced plans to add 33 of Boeing's 737 aircraft to its fleet.

Also in the running was McDonnell Douglas Corp.'s DC9 Super 80 aircraft.

In what is believed to be the largest arrangement of its kind, Boeing will sell the 33 aircraft to General Electric Credit Corp., a major capital financing organization, which in turn will lease the planes to Delta. The amount of lease payments was not disclosed. The price of a new 737 ranges from $15 million to $18 million.

The new 737s will be delivered to Delta during 1983 and 1984 -- five the first year and 28 the second -- under 15-year operating leases. The arrangement, which is subject to virtually certain approval by Delta's directors next month, allows Delta the option of returning any of the aircraft after eight years if it acquires the l50-seat new-technology aircraft it is urging the airframe industry to build, or if it buys other Boeing planes.

For Boeing, whose Commercial Airplane Co. has been suffering as a result of the worldwide recession and poor financial condition of many of the world's airlines, the Delta order means between $500 million and $600 million in revenues. The sale brings Boeing's orders for all types of commercial planes to 100 for the year, the lowest level in 10 years.

Delta officials said yesterday that the competition between the 737 and the DC9-80 went "right down to the wire." The 737 won out in the end primarily on two grounds, they said. The Boeing plane will be powered by the same Pratt & Whitney engines that Delta is using on its 727 fleet, making maintenance and the spare parts inventory more efficient; the DC9-80 uses a different engine.

The twin-engine 737 aircraft, which will carry 107 passengers, suited Delta's needs better than the 142-seat DC9-80, Delta officials said. The DC9-80 is similar to Delta's 727s, even though the DC9-80 is more efficient, the company said. Delta President David C. Garrett said yesterday the 737 will allow Delta to increase frequencies of flights on its short and medium-length routes in and out of its major hubs and will permit it to overfly the hubs and provide nonstop service in smaller markets.

Under Delta's pact with Boeing, the Seattle-based company also agreed to purchase 11 of Delta's older Lockheed L-1011 aircraft during the next two years.