Lockheed Martin Corp. LMT, -0.20% said Chief Executive Marillyn Hewson is stepping down after six years in the role, and will be succeeded by James Taiclet, effective June 15. Taiclet, 59, a Lockheed board member since 2018, has served as CEO of American Tower AMT, -2.08% since 2003. Hewson, 66, will become executive chairman of Lockheed. "I know it is the right time to transition the leadership of Lockheed Martin," Hewson said. "The corporation is strong, as evidenced by our outstanding financial results last year and a record backlog of business." The aerospace and defense company said Frank St. John, the executive vice president of the rotary and mission systems business will serve as chief operating officer. Lockheed's stock, which tumbled 10.3% in premarket trading amid a broad-market selloff, has slid 25.0% over the past month through Friday, while the S&P 500 SPX, -1.11% has declined 19.8%.