Blackstone Group Group said Monday it is buying the 110-story building . Formerly known as the Sears Tower , the building is the tenth tallest in the world . It was renamed Willis Tower in 2009 after Willis North America , an insurance brokerage that is a tenant in the tower , bought naming rights . Purchase adds 3.8million square feet of space to New York-based Blackstone Group 's huge investment portfolio .
Blackstone Group said Monday it is buying the 110-story building . Formerly known as the Sears Tower , the building is the tenth tallest in the world . It was renamed Willis Tower in 2009 after Willis North America , an insurance brokerage that is a tenant in the tower , bought naming rights . Purchase adds 3.8million square feet of space to New York-based Blackstone 's huge investment portfolio .