Shares of American Airlines were down 2 percent in extended trading, while United shares gained more than 1 percent.
In a letter to United employees, CEO Oscar Munoz said he appointed Kirby as president of the company, a newly created position. Munoz said the move is "the culmination of the formation of my leadership team."
"While his credentials speak for themselves, what set Scott apart from the other impressive candidates I considered was his excitement at the passion and enthusiasm he is seeing from all of you over the past few months. Simply put, he 'gets' the new spirit of United that you have created, and he is the right person to help all of us seize the opportunities ahead," Munoz said.