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Marillyn Hewson, the CEO of Lockheed Martin, the nation’s largest defense contractor, announced Monday that she would step down from her role effective June 15.

The 66-year-old Hewson, who will become the company’s executive chairman, will be replaced as CEO by James Taiclet, currently the chief executive of American Tower, a real estate investment trust.

Having held the top job at Lockheed Martin since the beginning of 2013, Hewson is not only one of America’s longest-serving CEOs, but one of the highest-ranking women in global business. Fortune ranked Hewson No. 1 on its Most Powerful Women list for the past two years in a row, above other female chief executives of Fortune 100 companies including General Motors’ Mary Barra and IBM’s Ginni Rometty, who recently announced she would retire in April.

“The timing is right not only for our company, but for me personally,” Hewson wrote in a LinkedIn post about her departure. “I have worked full time since I was 16,” she added, including at Dairy Queen as a teenager, and as Lockheed’s CEO since the beginning of 2013. “After the financial success of last year and the record $144 billion of business in backlog, the timing is right for this leadership transition,” the company told Fortune in a statement.

While Lockheed Martin would not say when Hewson gave the board notice that she would step down, a person close to the company said her decision was not related to the coronavirus outbreak nor the market’s reaction to the viral pandemic, nor did it come as a surprise. Hewson is still working from Lockheed’s Maryland offices, the person added, even as more companies send their employees home amid efforts to contain the spread of the virus.

The board’s decision to choose an outsider—and a male one—to succeed Hewson was somewhat unexpected. In an interview in 2018, Hewson told Fortune that Lockheed Martin had built a “strong pipeline” of female leaders who could potentially run the company one day, specifically citing Michele Evans, Lockheed’s head of aeronautics.

Evans has qualities that “I think [are] what you look for, whether it’s a man or a woman, to take over as CEO,” Hewson told me at the time. The rise of female talent across the defense industry “has got people to a point where now, there’s several [women] to select from,” she added. (Evans underwent treatment in the fall for what she told employees was “a significant medical issue”; she remains head of the aeronautics division.)

Hewson is one of 38 women who are currently, or are soon to be, CEOs of Fortune 500 companies, though that number may drop to 35 this spring with her and others’ departures. She’s also one of several high-ranking female leaders in the aerospace and defense industry, including Northrop Grumman CEO Kathy Warden, General Dynamics CEO Phebe Novakovic, and Leanne Caret, who heads Boeing’s defense, space, and security division.

Presiding over Lockheed Martin for more than seven years, Hewson helped build the company’s flagship F-35 fighter jet program into a powerhouse; forged a $9 billion acquisition of Sikorsky, the maker of Black Hawk helicopters; and expanded into emerging technologies such as hypersonic missiles, leading Lockheed to win billions of dollars in new contracts. She has also helped the company weather everything from criticism from President Trump (often delivered via public tweets) to conflicts in Syria and national security threats—both on the ground, in space, and in cyberspace—from North Korea, Russia and China in recent years.

Lockheed Martin stock fell more than 12% Monday—about the same as the broader market—and is down 35% from its February peak. But over the course of Hewson’s tenure, the stock has risen 212%, compared to about 67% for the S&P 500.

Lockheed Martin CEO Marillyn Hewson, Fortune’s most powerful woman in business, will step down

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