United Airlines plans a major increase in service from Dulles Airport by year-end -- a move that will bolster its dominance of the rapidly growing airport and make United the largest airline in the Washington area.

Just six months ago the picture at Dulles looked considerably different. Continental Airlines, which competes against United with major hubs at Dulles and in Denver, announced that it was absorbing New York Air, which had used Dulles as a base of operations. At the time Continental said it would increase its departures from Dulles to 97 a day, a move that would have made Continental the area's largest air carrier.

But Continental, which registered a second-quarter loss and and has been battling a reputation for poor service, announced last week that it was reducing its presence at Dulles from 80 flights a day to 50 or so.

John Zeeman, United's executive vice president for marketing and planning, said yesterday that United plans to increase daily departures from Dulles from 85 now to 110 by the end of the year. The airline will add service from Dulles to between five and 11 new cities on the East Coast. The final selection of cities has not yet been made, he said.

With the expansion, United will have s total of 131 departures a day from the Washington area's airports -- Dulles, Washington National and Baltimore-Washington International. Continental has 106 departures a day from the three airports.

"The idea all along has been to become the leading carrier at Dulles," said Zeeman. "When we went in there we had a lot of competition, including New York Air and Presidential." Presidential Airways, which had hoped to dominate Dulles, instead sold its terminal to Continental and entered into a marketing arrangement under which Presidential provides feeder service called "Continental Express."

Dulles' importance as a hub airport reflects the changed nature of an industry in which airlines route as many flights as possible through such airports. Individual carriers increasingly dominate "hub" airports where many flights converge in a pattern that increases the airline's traffic. An airline might not be able to justify more than one flight a day from city A to city B. But it might be able to justify eight flights a day from city A to its hub, where passengers could connect with flights to 30 different ultimate destinations, including city B.

United, which also has hubs in Chicago, San Francisco and Honolulu, has been locked in fierce competition with Continental at Denver as well as Dulles. Continental is the largest carrier in Denver measured by daily departures, although United claims that it is carrying more passengers from the Colorado airport.

Continental has hubs in Newark and Cleveland that serve some of the same markets it has served from Dulles. "They can almost get the same things accomplished out of those markets" that they could from Dulles, said Paul P. Karos, an airline industry analyst with L.F. Rothschild.

Continental said that it is cutting back its Dulles service by about a third. "What we are doing is to focus on Washington as a local market and not to concentrate so much on the connectability of it," said Doug Birdsall, senior vice president for marketing and planning at Continental. Birdsall, who once headed New York Air and helped build Continental into a major hub for the airline, noted that the Washington area is a strong market by itself. Some of the connecting traffic that Dulles once served can be served just as efficiently from Newark, he said.

The decisions by the two airlines reflect the fact that United "has made some major market share gains, and {are} a sign to me that Continental isn't going to continue to make a huge commitment to the Dulles market," Karos said. United needs the Dulles hub to provide service up and down the East Coast that it cannot offer conveniently from its other hubs, he said.

Zeeman said that United is expanding its midfield terminal at Dulles in cooperation with Trans World Airlines to provide five additional gates. That project is expected to be completed by May 1988, he said. United had record boardings at Dulles in April, when it boarded about 200,000 passengers. Dulles is still United's smallest hub, however.