With Friday's news that Marillyn Hewson will take the helm at Lockheed Martin (LMT), women in corporate America are about to hit a milestone. On January 1, there will be 21 female Fortune 500 CEOs.

That's a record, and one more than the U.S. Senate, which increased its female representation by two, to a record 20 female Senators, in last week's elections.

Granted, it's not quite time to pop the champagne. After all, 21 female CEOs represent a paltry 4.2% of Fortune 500 chiefs. Nonetheless, it is significant as well as surprising that the women rising to the top these days are different from women leaders of the past, who tended to make it in industries like retail, beauty and consumer packaged-goods.

At the end of this year, Phebe Novakovic is due to take charge of another giant defense contractor, General Dynamics (GD).  Meanwhile, America's two largest tech companies appointed female chief executives in the past year or so: Meg Whitman at Hewlett-Packard (HPQ) and Ginni Rometty at IBM (IBM). Another of this year's high-profile CEO placements: Marissa Mayer at Yahoo (YHOO).

And leading Fortune 500 wannabes—that is, companies ranked 501 to 1000 on Fortune's annual list of America's largest companies—are 21 more women. These female chiefs are leading multibillion businesses in oil, energy, and steel—fields that are obviously not just for the boys anymore.