The airline world has a new No. 2 after United Continental Holdings Inc.’s aggressive domestic expansion pushed the carrier ahead of Delta Air Lines Inc. in terms of passenger traffic.

Early last year, United embarked on an effort to regain market share at its three mid-continent hubs in Chicago, Houston and Denver. Under a three-year push, United is targeting capacity increases of as much as 6 percent annually as it seeks to increase flight connections—and profits—at its busiest airports.

United last year recorded 230.2 billion revenue passenger miles, an industry yardstick of traffic that multiplies the number of passengers by the distance flown, the company said Wednesday in a statement. Delta had 225.2 billion, according to a report last week. Delta took over the No. 2 spot in 2015 as Chicago-based United retrenched.

“Our goal is not to be the biggest. We want to be the best and as we implement our strategy, we are looking to build on this momentum in 2019,” United spokeswoman Megan McCarthy said in an email.

American Airlines Group Inc., which has yet to release its traffic for all of 2018, is expected to retain its title as the world’s largest carrier.

United also led major U.S. carriers in stock gains last year, with a 24 percent jump. The other eight companies on a Standard & Poor’s airline index all lost value.