The Defense Department said Friday the Marine Corps' F-35B variant will reach initial operational capability (IOC) by December 2015, while the Air Force's F-35A variant will follow a year later and the Navy's carrier variant is set to reach the milestone by February 2019. Congress directed the Air Force and Navy secretaries to provide a report by Saturday that details the IOC dates for the three variants of the Joint Strike Fighter. DoD said in a statement the Air Force, Marine Corps and…