Lance Armstrong greets American Century Investments founder James Stowers Jr. in 2006. The Lance Armstrong Foundation announced Livestrong Portfolios, a series of mutual funds designed to simplify investing. (ORLIN WAGNER/ASSOCIATED PRESS)

James E. Stowers Jr., the billionaire founder of one of the nation’s leading investment management firms who gave away most of his fortune to fight disease, died March 17 at his home in Kansas City, Mo. He was 90.

His firm, Kansas City-based American Century Investments, announced the death but did not disclose the cause.

Mr. Stowers was a struggling mutual fund salesman in 1958 when he founded Twentieth Century Investors with only two mutual funds and $107,000 in assets. That company grew into American Century Investments, one of the nation’s leading investment management firms, and currently manages about $141 billion.

In 2000, Mr. Stowers and his wife, Virginia, who both successfully fought cancer, promised more than $1 billion of their fortune to create the Stowers Institute for Medical Research in Kansas City. The endowment eventually grew to $2 billion.

“I’ve always said that if I make other people successful, they’ll make me successful,” he told the Associated Press in 2009. “We wanted to give something that was more valuable than money to the millions of people who made our success possible.”

The institute has attracted world-renowned researchers to Kansas City and prompted civic leaders in the region to enhance collaboration between area research groups, health-care organizations, universities and business interests.

Mr. Stowers and his wife said they wanted the institute to focus on basic research into how genes work in hopes of finding ways to fight such ailments as cancer, heart disease and Alzheimer’s disease. But they faced skepticism from well-known research institutions about whether the Kansas City area could be a major player in biomedical research and the life sciences.

By 2013, the institute had about 370 scientists, research associates, technicians and support staff members.

“We were told that what we wanted to do couldn’t be done here,” Mr. Stowers said in the 2009 interview. “We had a whole bunch of labs tell us that they could do a better job than us.

“You know, I had the same statements made to me when I started American Century. I just said, ‘I can prove it; I can do this.’ ”

James Evans Stowers Jr. was born Jan. 10, 1924, in Kansas City. He served in the Army Air Forces as a fighter pilot during World War II, then earned a degree in medicine from the University of Missouri.

He and his wife, the former Virginia Glascock, said they were motivated to help find cures by their bouts with cancer. Mr. Stowers was diagnosed with prostate cancer in 1987, and his wife had surgery for breast cancer in 1993.

Mr. Stowers’s efforts pushing stem-cell research sparked controversy in 2006 when state voters narrowly approved a constitutional amendment guaranteeing that any federally allowed stem-cell research and treatment could occur in Missouri.

The Stowerses donated most of the $30 million spent by the Coalition for Lifesaving Cures, an organization formed to support the amendment, saying stem-cell research was important to the institute’s efforts to fight disease.

The amendment was narrowly approved after a bitter political fight in which opponents claimed that the use of embryonic stem cells for research required the destruction of a human embryo. Supporters contend the research does not end a life and had the potential to cure many debilitating diseases.

Besides his wife, survivors include three children; a brother; and many grandchildren. A daughter died in 2010.

— Associated Press