Industrialist Herbert Quandt and his wife, Johanna, in 1971. (AFP/Getty Images)

Johanna Quandt, the billionaire widow whose husband saved Germany’s BMW from bankruptcy half a century ago and laid the foundations for the company to become the world’s largest maker of luxury cars, died Aug. 3 at her home in Bad Homburg, Germany. She was 89.

The Johanna Quandt Foundation announced the death but did not disclose the cause.

The third wife and onetime secretary of Herbert Quandt, Johanna Quandt inherited a 16.7 percent stake in Munich-based BMW, the largest single holding in the company, when her husband died in 1982. Her shareholding in the car maker will remain in the family, foundation spokesman Joerg Appelhans said.

Mrs. Quandt also owned a stake in Datacard Group, a closely held Minnetonka, Minn.-based credit-card and passport maker, and held shares in Gemalto, a publicly traded security-software designer based in Amsterdam.

Mrs. Quandt’s net worth of $11.5 billion ranked 98th in the Bloomberg Billionaires Index and eighth within Germany. BMW has a market value of $65.4 billion.

BMW, which competes with Daimler’s Mercedes-Benz and Volkswagen’s Audi in the premium-vehicle market, had sales of 80.4 billion euros in 2014 and delivered a record 2.12 million cars worldwide. Its brands include BMW, Mini and Rolls-Royce. Mrs. Quandt and her two children held a combined 46.8 percent of the company.

Mrs. Quandt, who rarely spoke to the media, remained on the company’s supervisory board until 1997, when she stepped down. “I appeal to the people in my company, and not to the general public,” she was cited as saying in the Frankenpost newspaper in 2012.

In 2007, a television documentary explored the family’s ties to the Nazi regime during World War II. Johanna Quandt’s father-in-law, Guenther Quandt, was contracted to make Mauser firearms and anti-aircraft missiles for the Third Reich’s war machine.

In response to the broadcast, the family commissioned a Bonn-based history professor, Joachim Scholtyseck, to examine the extent of the involvement. The study showed that forced labor was used in Quandt factories during the war.

Johanna Bruhn was born in Berlin on June 21, 1926. Her father, Wolfgang, was an art historian.

After attending school in Potsdam and Berlin, she began an apprenticeship in medical technology, but her training was interrupted by the war. She found her first job as a banker’s secretary in Cologne before joining Herbert Quandt’s office in Bad Homburg, a spa town near Frankfurt, in the mid-1950s.

At the time, he and his half-brother, Harald Quandt, had just inherited a portfolio of about 200 companies after their father, Guenther Quandt, died in 1954. The holdings included the battery producer Varta; arms producer Deutsche Waffen-und Munitionsfabriken, which became Industrie-Werke Karlsruhe after the war; a stake in potash miner Wintershall; and shares in Stuttgart-based car maker Daimler and BMW.

Within a few years, Johanna Bruhn became Herbert’s personal assistant, with an increasing influence over his business decisions. They married in 1960 and had two children, Susanne and Stefan, who survive.

In 1959, against the advice of his bankers, Herbert Quandt was swayed by employees and some smaller stakeholders to boost his stake in the almost-bankrupt BMW to 50 percent to fend off a takeover attempt by Daimler. He aimed to turn the company around with new models, such as the BMW 1500 “sporty sedan.” The rescue plan, implemented the year he married for a third time, saved the automaker from collapse.

In 1995, Mrs. Quandt set up her own foundation, which supports young people training to become business journalists and awards a media prize each year. She also provided funding to help children with cancer and financed cultural groups that staged art exhibitions. In 2012, she committed as much as 40 million euros over a 10-year period to a Berlin-based institute for health-care research.

— Bloomberg News