Marlboro was originally branded as a woman's cigarette.
Filtered cigarettes were considered feminine as reflected by Marlboro's original slogan "Mild as May." In the 1930s, Marlboro even changed the cigarette tips from ivory to red so they wouldn't smear ladies' lipstick.
In 1955 Philip Morris & Co. tried to change Marlboro's feminine image with the "Tattooed Man" campaign, where a rugged cattle rancher, a Navy officer, and a flyer (all with muscular, tattooed hands) were shown holding a cigarette. Supposedly the tattoo was suggestive of "romantic past." Later, ad genius Leo Burnett used the image of a cowboy to prove that the cigarettes weren't for sissies, and thus "The Marlboro Man" was born. (Source)
There were many Marlboro Men, but two of them - Wayne McLaren and David McLean - died of lung cancers. The original Marlboro Man, David Millar, Jr., died of emphysema. McLaren became an anti-smoking activist before he died, and Philip Morris denied that he ever appeared in a Marlboro ad. Later, the company admitted that he indeed appeared, but not as a Marlboro Man.