Today, Bill Gates is one of the biggest philanthropists in the world.

Through the Bill & Melinda Gates Foundation, he donated a total of $1.5 billion in 2014 alone, mostly towards efforts to improving health and education around the world.

Gates credits his philanthropic spirit to his late mother Mary, who died of breast cancer in 1994. 

Microsoft's 1986 IPO made 30-year-old Gates a billionaire. According to a 2009 Wall Street Journal article, after he struck it rich, Mary urged him to give some of the money away.

"I'm just trying to run my company!" he reportedly said to her, not wanting to be distracted from his work at Microsoft.  

She eventually convinced him to start a program at Microsoft that would help raise money for the United Way, whose board he would join in later years. May was the first female president of King County's United Way and served as the director of First Interstate Bancorp, U S West Inc., and KIRO-TV of Seattle.

Gates has often spoken about a letter Mary wrote his then-fiance Melinda the day before their wedding. 

"From those to whom much is given, much is expected," the letter read. Mary passed away six months later.

Gates still has the letter today. 

A few months after his mother's death, Gates, with the help of his father, allocated about $100 million for what would become the Bill & Melinda Gates Foundation. 

--This story first appeared on Business Insider.