FILE PHOTO: Fourth F-35 Lightning II arrives at Nellis Air

FILE PHOTO: The fourth U.S. Air Force F-35A Lightning II aircraft arrives at the 422nd Test and Evaluation Squadron at Nellis Air Force Base, Nevada in this April 24, 2013 photo released on May 8, 2013. Picture taken April 24, 2013. Daniel Hughes/U.S. Air Force/Handout via Reuters