DALLAS – American Airlines and US Airways will merge and supplant United as the world’s biggest airline.

The boards of both companies approved the merger late Wednesday, according to four people close to the situation.

The carrier keeps the American Airlines name but will be run by US Airways CEO Doug Parker. American’s CEO, Tom Horton, will become chairman of the new company, the sources said. They requested anonymity because the merger negotiations were private.

A formal announcement is expected Thursday morning.

The deal has been in the works since August, when creditors forced American to consider a merger rather than remain independent.

American has been restructuring under bankruptcy protection since late 2011.