Former Vice President Dick Cheney kept a secret resignation letter locked in a safe in case he ever became incapacitated while in office.

Cheney wrote the letter because of his history with health problems. He had suffered four heart attacks when he signed the letter in March 2001. He suffered another in 2010.

In an interview with NBC News to help promote his forthcoming memoir, "In My Time," Cheney said that only two other people knew of the letter's existence. President George W. Bush was one. The other was a member of Cheney's staff.

"I did it because I was concerned that--for a couple of reasons," Cheney told Jamie Gangel in an interview that will air August 29 on "Dateline." "One was my own health situation. The possibility that I might have a heart attack or a stroke that would be incapacitating. And there is no mechanism for getting rid of a vice president who can't function."

Cheney hinted that his book may not be entirely kind to former President George W. Bush. "I didn't set out to embarrass the president or not embarrass the president," Cheney said. "If you look at the book, there are many places in it where I say some very fine things about George Bush. And believe every word of it."

Cheney's book will be published on August 30.