United Airlines will expand on the East Coast.

United President James J. Hartigan said the carrier, the nation’s largest, will establish a major hub at Dulles International Airport near Washington and from there run at least 50 departures a day to 24 cities beginning May 6. The number of flights is expected to increase to more than 100 a day within three years, he said. The airline already serves several East Coast cities and has 16 flights a day out of Dulles, but it has avoided the tight competition for travelers flying between the Northeast and Florida that has pushed fares to as low as $39.