Northwest Orient Airlines will add significantly to current service at Dulles International Airport on May 24 when it begins offering the airport's only round-trip flights to Chicago.

The airline said it will offer three flights to Chicago and two flights back each day at fares up to 63 percent lower than most current coach fares to Chicago from National Airport.

The unrestricted one-way coach fare will be $87 on weekday flights and $69 on weekends. The current one-way coach fare to Chicago's O'Hare International from National Airport is $189, with a fare of $109 available for some seats on certain flights. Midway Airlines' unrestricted fare from National to Chicago's Midway Airport is $109.

Until July 1, Northwest will offer a first-class one-way fare of $99 compared with the $270 first-class fare of its competitors from National Airport. Northwest said its first-class fare will rise to $170 in July.

Northwest's flights will provide a new test of the popularity of Dulles, whose supporters say it would attract large numbers of travelers if more flights were available there.

Chicago is the third-largest air travel market from the Washington area, surpassed only by New York and Boston. But now, out of 25 area nonstop flights to Chicago, only one--a Northwest flight--leaves from Dulles; the flight doesn't return to Dulles, however.

"We've always said we support both National and Dulles and were prepared to put in more service at Dulles if the market supports it," William A. Kutzke, Northwest's vice president-Washington, said yesterday. "We're going to give it a real shot, a real market test.

"We're coming in with good flight frequencies, low fares and a strong advertising campaign to inauguarate the service," he said. "We're going to do everything possible to promote it."

The new service undoubtedly will be watched carefully by officials of other airlines who express occasional skepticism about claims of Dulles' potential.