Armstrong underwent a heart-bypass surgery earlier this month, according to Reuters.
The American astronaut led the Apollo 11 mission and on July 20, 1969, planted his left foot on the moon's surface, famously saying, "That's one small step for a man, one giant leap for mankind," according to ABC News.
He spent nearly three hours walking on the moon with crewmate Edwin "Buzz" Aldrin that day.
Armstrong stayed out of the spotlight in the following years, but spoke in February at an event honoring fellow astronaut John Glenn, NBC News noted.
Before the moon landing, Armstrong had served as a test pilot and a Naval aviator during the Korean War, ABC News said. A native of Wapakoneta, Ohio, Armstrong had an engineering degree from Purdue University.
In 1994, at a rare appearance at the White House on the Apollo 11 landing's anniversary, Armstrong said, "There are great ideas undiscovered, breakthroughs available to those who can remove one of truth's protective layers," according to ABC News. "There are places to go beyond belief."