Blackstone Group LP has struck a deal to purchase the Willis Tower in Chicago for $1.3 billion in what would be the highest price ever paid for a U.S. office tower outside of New York, according to executives of the private-equity firm.

The deal for the formerly named Sears Tower, reached over the weekend, would be one of the highest-profile towers bought by Blackstone, which is better known for leveraged buyouts of chains like Hilton Worldwide Holdings Inc. than for individual property purchases.

...