Delta Air Lines said Tuesday it will offer broadband wireless Internet access on its entire domestic mainline fleet by the middle of next year. The Atlanta-based company said WiFi service will be offered for a fee to customers traveling throughout the continental U.S. The nation’s No. 3 carrier is partnering with Aircell, an airborne communications provider, to install the network on Delta’s domestic fleet of more than 330 aircraft. The system will allow Delta customers traveling with WiFi enabled devices — such as laptops, smartphones and PDAs — to access the Internet while in flight.