After dropping off the list in 2013, OWN CEO Oprah Winfrey has made her way back into the ranks, thanks to her 2015 investment in Weight Watchers. The partnership helped to take Weight Watchers stock from $6.79 in 2015 to a high of $103 in June, placing Winfrey, already a billionaire, among the world's 500 richest people .

The 51-spot list includes seven newcomers and two returnees. Lockheed Martin's chairman, CEO and president, Marillyn Hewson, takes the top spot. General Motors CEO Mary Barra ranked No. 2, having previously held the No. 1 spot since 2015.

While Winfrey's return is worthy of applause, she is the only black CEO who makes the list. Altogether, women of color comprise just 7.8 percent of the Most Powerful Women List — along with Winfrey, PG&E Corporation's CEO Geisha Williams, Home Depot's Ann-Marie Campbell and Starbucks' Roz Brewer make the list.

It speaks volumes about the broader business landscape and striking lack of diversity at the top. The number of female CEOs at Fortune 500 companies dipped from 32 last year to 24 this year (women lead less than 5 percent of the companies on the list) and includes no black women. When PepsiCo's CEO Indra Nooyi steps down from her position this year, Williams will be the only remaining woman of color to lead a Fortune 500 company.

But Fortune's list highlights the strides that have been made in aerospace and defense. Four of the top five U.S. defense businesses will be led by women by the end of the year: Lockheed's Hewson, Phebe Novakovic from General Dynamics and Leanne Caret from Boeing. When Northrop Grumman CEO Wes Bush steps down at the end of 2018, he will be replaced by the company's current COO Kathy Warden.

Tom Kennedy, the chairman and CEO of Raytheon (and the only leader of a top five U.S. defense business who isn't on this list) tells Fortune, "I couldn't be prouder of our industry."

Hewson, who took on her leadership position at the same time that Novakovic stepped into hers in 2013, tells Fortune that the two women have discussed the impact of their executive roles, and hope they will inspire other women to say, "I can be in that job."

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