The already small roster of Fortune 500 companies led by women will shrink even further after PepsiCo Inc. Chief Executive Indra Nooyi steps down from her role this year.

But in one sector that has been male-dominated, gender balance seems to be moving in the opposite direction.

By next year, three of the top U.S. defense firms — Lockheed Martin Corp., General Dynamics Corp. and Northrop Grumman Corp. — will have a female CEO. Boeing Co. has had a woman at the helm of its $21 billion defense, space and security business since 2016.

While industry officials and observers have cheered the progress, many say the corner office moves don’t reflect sweeping change in the industry. A survey released last year by trade publication Aviation Week found only 24 percent of the aerospace and defense workforce is female.

That number is down from 26 percent 10 years earlier.

Women made up only 3.2 percent of the logging industry, 9.1 percent of the construction workforce and 23.5 percent of the transportation and utilities industry, according to a report published last year by Catalyst, a nonprofit that supports women in the workplace.

“Half of the population in the world today is women,” said Leanne Caret, chief executive of Boeing’s defense, space and security sector. “If we want to take full advantage and be the company we can, then we need to have full access and availability to the talent globally. And why would we ever want to limit ourselves to a portion of the population?”

Northrop Grumman is the latest defense company to name its first female CEO, announcing last month company President Kathy Warden will move into the top job next year. She will join CEOs Marillyn Hewson of Lockheed Martin and Phebe Novakovic of General Dynamics.

Hewson, Novakovic and Caret were named to Fortune’s list of most powerful women in business last year. Nooyi of PepsiCo made the list, as well as Mary Barra, CEO of General Motors Co., and Denise Morrison, then-CEO of Campbell Soup Co.

“It’s a lot of the larger companies and heavier industry today where the meritocracy reigns supreme again,” said Jeffrey Sonnenfeld, senior associate dean at the Yale School of Management.

Warden, like Hewson, Caret and Novakovic, steadily ascended the ranks over the years since joining Northrop Grumman in 2008 as vice president of strategic intelligence initiative.

In the last 10 years, she served as president of the company’s software-focused mission systems sector and chief operating officer.

That upward trajectory into leadership positions is important if women are to advance within companies, industry observers said. The growing number of female CEOs across the defense industry indicates the pool of women engineers that joined the workforce decades ago are finally reaching senior positions.

“You almost have to get to the point where you have women in executive vice president and senior vice president positions in order for them to demonstrate how successful they will be,” said Rachel McCaffrey, executive director of Women in Defense.

Companies can eliminate barriers that hold women back, said Anna Beninger, senior director of research and corporate engagement partner at Catalyst. Northrop Grumman and Lockheed Martin recently won the Catalyst award for their work to increase workplace diversity.

“It’s not surprising to me, honestly, that you now have women leading these companies, because they’re really working to create opportunities for women across the board,” Beninger said.

Increased diversity on a company’s board of directors is a factor in increasing the number of women in the C-suite as the board signs off on CEO candidates, McCaffrey said.

The increased number of female defense CEOs could help recruit and retain women, as it shows career advancement is possible.

“Part of my responsibility is to open the doors for those that come after us, men and women,” said Caret of Boeing.

Caret is a second-generation Boeing employee; her parents met at a facility outside New Orleans where they were building the Saturn V rocket that took astronauts to the moon.

She joined Boeing in 1988 and served as chief financial officer for the defense, space and security division, and vice president of Boeing’s rotorcraft programs, and led the firm’s global services and support business.

Caret noted she, Hewson, Warden and Novakovic built a track record by succeeding despite tough assignments.

“These aren’t positions you get to because someone handed them to you. We’ve earned them just like everyone else,” Caret said.

The industry has made strides since Linda Hudson was dubbed the “first lady of defense” by Washingtonian magazine after she was named president and CEO of the U.S. subsidiary of BAE Systems in 2009.

Some of the industry’s notorious C-suite scandals hinted at “boys’ club” culture. Former Boeing CEO Harry Stonecipher resigned in 2005 after he had an affair with a female executive. In 2012, incoming Lockheed Martin CEO Christopher Kubasik resigned after an investigation confirmed he’d had a “close personal relationship” with a subordinate.

He was replaced by Hewson, who became Lockheed Martin’s first female CEO.

Recently, the editor of trade publication Defense News wrote in an op-ed more than 200 women from the national security sector recently signed a #MeToo letter describing themselves as survivors of sexual harassment, abuse or assault, or knew others that were.

“We’re not there yet,” Jill Aitoro wrote.

Beninger of Catalyst said more needs to be done to ensure better representation of women of color in the workforce and in leadership ranks. And work has been done to increase awareness of science, technology, engineering and math careers as early as elementary school, said Robin Thurman, director of workforce policy and industrial base development at the Aerospace Industries Assn. trade group.

“The fact that these women have been able to be successful does say a lot about these organizations,” Beninger said of defense firms. “But they do have a ways to go in terms of reaching parity.”