How United is trying to overtake Delta as the world's most profitable airline

United Airlines is the best-performing U.S. airline stock this year. It wants to be the most profitable carrier, too.

United shares are up more than 80% since Jan. 1, more than double the gains of Delta Air Lines, the country's most profitable airline.

United has been flying for nearly a century and is the world's largest airline by capacity. The Chicago-based carrier hasbeen trying to expand its profits by investing heavily in roomier seats and growing a network that will excite high-spending travelers, with routes going beyond vacation bread-and-butter destinations and into places like Greenland and Mongolia.

"We've been able to offer more seats at more price points, and that has allowed us to compete with the low-cost carriers in a price-effective way for the the basic economy traveler," said Patrick Quayle, United's senior vice president of global network planning and alliances. "But it also allows us to compete with the foreign flag carriers by offering a better product in the front of the cabin."

United has the most international routes of any U.S. airline.

"We still have not achieved that domestically. We're still, you know, No. 3 or 4 depending on how you measure it," said Andrew Nocella, chief commercial officer of United Airlines. "But we think we have a lot of runway in front of us there as well. So we'll see where we are in a few years."

The top two U.S. airlines are locked in an arms race to win over customers.

"United Airlines is an airline that is trying to be a more premium airline right now, much as Delta has been for more than 10 years," said Henry Harteveldt, president of Atmosphere Research Group, a travel industry consulting firm.

United is outfitting hundreds of planes with Bluetooth connections, new