The Chicago-based Star Alliance carrier will offer six daily flights to Denver and San Francisco from the airport, which is 37km north of downtown Seattle, from August 2018, it says in a letter to employees on 10 August.
In May, Alaska Air Group said it will begin passenger service, also in fall 2018, with tickets going on sale early next year for nine daily departures to destinations along the West Coast from OR to California.
Flights to the airport, located just over 30 miles from larger Seattle-Tacoma International Airport, are meant to provide passengers living in North Seattle and Northwest Washington an easier option when looking to fly, according to United Vice President of Worldwide Sales Dave Hilfman.
United follows Alaska Air Group in committing to scheduled flights out of the Snohomish County airport, ensuring that the planned passenger terminal will operate nearly at full capacity as soon as it opens next year.
"As far as capacity goes we are getting pretty close", said Propeller Chief Executive Brett Smith. A 2012 Federal Aviation Administration decision capped daily flights at almost two dozen.
United's new flights to Paine will complement the 36 it offers to Seattle Tacoma, it says.
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Paine Field initially served as an alternate landing site on days when weather prevented commercial flights from landing in Seattle, as well as a hub for small private airplanes.
United has not yet specified the types of aircraft it would fly out of Paine Field, which now sees more than 300 flights per day, 96 percent of which come from general aviation and aerospace activity.
Paine Field has since become the center of Boeing's production facilities.
Paine Field is now dominated by one of two Boeing aircraft manufacturing plants. The airport now plays host to the largest building in the world by volume, used to manufacture numerous Boeing aircraft, from the 747 to the 787, among others.
While many expect commercial service at Paine Field to reduce southbound traffic on Interstate 5 to Sea-Tac airport by hundreds of cars per day, residents of neighboring Mukilteo have expressed concern about increased traffic on city streets, along with increased noise and the potential for decreased property values.