Marillyn Hewson, a nearly 30-year Lockheed veteran, currently runs the company's largest division.

By Beth Kowitt, writer

FORTUNE -- The fall of Lockheed Martin's incoming CEO Christopher Kubasik has elevated one of Fortune's Most Powerful Women.

Marillyn Hewson, No. 19 on our power list this year, was due to become COO on January 1 when Kubasik took over the top job from Bob Stevens -- that is until today when Kubasik resigned after an ethics investigation showed he had a relationship with a subordinate. Now Hewson will take the helm at the start of the year.

Hewson, a nearly 30-year Lockheed (LMT) veteran, currently runs the company's largest division, Electronic Systems, which generated nearly a third of the company's sales in 2011.

At the Fortune Most Powerful Women Summit this summer, Hewson spoke on a panel about women in defense, during which she addressed the rising number of female executives in the industry moving into increasingly senior roles. (Phebe Novakovic is also slated to become a defense industry CEO when takes over at General Dynamics (GD) on January 1.)

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"I don't think it's coincidence at all," Hewson said. "I think it's a matter of the fact that the three of us [on this panel] and many of our colleagues have been in this industry for decades, and so we have decades of experience, we have decades of a track record of success, and we've moved up in the organization to compete for the top jobs."

Even before today's announcement, she was already one of the highest-ranking women in the industry. But it seemed that Hewson, 58, had reached her peak at Lockheed when her promotion to COO was announced in April. Kubasik at 51 had a long runway out in front of him.

Hewson, who grew up in a small town in Kansas, also talked about her roots in the defense world during the panel discussion. She lived adjacent to Fort Riley, her father was with the department of the army, and her mother was a nurse with the Women's Army Corps during World War II.

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She said when she went to interview at Lockheed in Marietta, Georgia, she was hooked after seeing all of the aircraft on the factory floor and realizing the challenges of the job. After that, "I never really wanted to work anywhere else," she said. "That's what seized my imagination."