Abstract : The F-35 Lightning II, also known as the Joint Strike Fighter (JSF), is the Department of Defense's (DOD) most costly and ambitious aircraft acquisition, seeking to simultaneously develop and field three aircraft variants for the Air Force, Navy, Marine Corps, and eight international partners. The JSF is critical for recapitalizing tactical air forces and will require a long-term commitment to very large annual funding outlays. The current estimated investment is $323 billion to develop and procure 2,457 aircraft. As required by law, this report discusses (1) program cost, schedule, and performance; (2) manufacturing results; and (3) test plans and progress. GAO's work includes interviews, cost data, test plans, production measures, and analyses by defense and contractor officials.