Willis Tower, Chicago’s tallest building, may soon have an equally big owner: Blackstone Group, the world’s largest private equity real estate owner.

New York-based Blackstone has a preliminary agreement to pay almost $1.5 billion for Willis Tower, according to a person familiar with its plans. Blackstone representatives have been visiting the building to perform due diligence on the 110-story icon, and it is looking to buy the West Loop tower for one of its funds…