​What drives GM CEO Mary Barra?

General Motors has manufactured more than 500 million cars over the years. The company is celebrating that milestone this week after some recent reverses ... and with a relatively new chief officer in the sdriver's seat. Anna Werner paid her a visit:

We met CEO Mary Barra on what for her is familiar ground: the Detroit-Hamtramck assembly plant she once managed.

"You used to run this plant -- what's it like to come back as CEO?" asked Werner.

"Well, it's a lot of fun," said Barra. "As I walked in, everybody said 'Welcome back.'"

She has spent her entire career with the company, and people like Ed Fleming look at her as one of their own.

"Real proud of the fact that I had interfaced with Mary early in her career, and then to see her make it to that level of the organization, was absolutely phenomenal for us," said Fleming.

Fifty-three-year-old Barra took over in January 2014, making her the company's fifth CEO in six years -- and the first woman to run a major automaker.

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Mary Barra (left), the first woman to run a major automaker, is on a journey to make General Motors a defect-free company. CBS News

Barra started at GM 35 years ago as an 18-year-old intern, then worked her way up from engineer to vice president, getting an MBA at Stanford along the way.

One of her jobs was head of human resources, where she was famous for cutting down GM's 10-page dress code to just two words: "Dress appropriately."

But Barra didn't envision her own future including the top job. "The CEO position was never something that, you know, five years or two years ago I said, 'Oh, that's what I want to do,'" she said.

"Never? Never thought about it?"

"Well, one of my core principles is whatever job you're doing, just own it and do it like you're going to do it the rest of your life," she said. "Because that's what I tell people: do your best and good things happen."

But not always. Just weeks into her new job, Barra faced one of the biggest challenges in GM's history: recalls that would eventually total more than 30 million vehicles.

The most significant recall: 2.6 million older Chevy Cobalts, Saturn Ions and other vehicles with suspected ignition switches that made airbags inoperable -- a defect now linked to 90 deaths and 163 injuries.

Barra was grilled by members of before Congress about what happened:

Senator Barbara Boxer: "Ms. Barra, I really hate to say this, but if this is the new GM leadership, it's pretty lacking. I am very disappointed, really, as a woman to woman, I am very disappointed, because the culture that you are representing here today is a culture of the status quo."