United Airlines is moving it's corporate headquarters to Chicago's Willis Tower.

The Chicago Sun-Times reports the deal United signed is one of the largest office leases ever in the city's history.

The deal extends United's lease through the year 2028.  Willis Tower already is home to the airline's network operatations center.

United will now occupy approximately 25 percent of the building.

The airline has been slowly moving its work force from Elk Grove into downtown since 2007.