Burger King, the second-biggest U.S. burger chain by store count, is introducing an application that will allow customers to pay for Whoppers with their smartphones as it races rivals to woo younger diners. The program will be introduced next month and should be in all of Burger King’s more than 7,000 U.S. locations in “a few months,” Bryson Thornton, a spokesman for the company, said in an e-mailed statement. The option to order food and drinks ahead of time for later in-store pickup may be added, he said. Fast-food chains including McDonald’s Corp. and Dunkin’ Brands Group are competing to quickly introduce the best loyalty programs and smartphone apps to try to attract millennials and teens. McDonald’s last year said it was testing coupon and mobile-payment apps at some of its U.S. locations. Dunkin’ Donuts rolled out a rewards programs to all of its domestic shops in January.