How to become CEO of a huge public company by Anne Fisher @FortuneMagazine October 9, 2015, 12:11 PM EDT E-mail Tweet Facebook Linkedin Share icons Note to Millennials, and anyone else with a frequent urge to change jobs: If you want to be CEO someday, pick an employer where you can see yourself staying for at least 10 years, or possibly a lot longer. Then get ready to hang in there for the long haul. Rumors of the death of employee loyalty have been greatly exaggerated, according to a new study from consulting firm Feigen Advisors. Almost 70% of the 29 chief executives appointed at the biggest 250 companies in the S&P 500 last year had been with their companies for at least a decade, and one-third had spent more than 25 years there. “Overall, nearly 90% of new CEOs were promoted from inside the company,” the report notes. These chiefs—including Satya Nadella at Microsoft, Mary Barra at GM, and Doug McMillon at Wal-Mart—run companies with a combined 5.3 million employees. Total revenues exceed $1.8 trillion, more than the GDP of India and nearly three times that of Switzerland. The firms’ “enormous economic clout,” says Marc Feigen, helps explain why boards of directors these days like to “go with the known quantity, [choosing] seasoned in-house leaders who have proven themselves for many years.” What’s the most common route to the top? Finance. “Serving as CFO enhances the odds of winning the top spot,” says the study, “underscoring directors’ increasing preference for leaders with financial expertise.” A background in engineering ran a close second. Don’t have an MBA? No worries. Neither do two-thirds of these CEOs. These larger companies are also “more willing to appoint a woman CEO, especially one who has risen through the ranks” than the rest of the S&P 500, the report notes, pointing out that 10% of the biggest enterprises now have CEOs who are women, versus just 4% for the S&P 500 overall. The female CEOs reached the top after working at their companies for an average of 26 years.