The originators are the penny dreadfuls (England) and dime novels (American) as well as newspaper serials (Like Wells' War of the Worlds). Dime novels often told embellished accounts of real people; such as Kit Carson, Wild Bill Hickhock, Jesse James, etc. Penny dreadfuls were British and heroes often feature Robin Hood type characters, highway robbers, etc. Newspaper serials were short parts of a longer work; in action/adventure, the best well known is King Solomon's Mines. It has become a classic. These lasted until the early 20th century.
These were published very cheaply, and very quickly.
Then came the pulps. (My personal favorite era)
The pulps were magazines printed on cheap paper made of wood pulp and told the tales of Western heroes, detectives, and eventually, the precursor to superheroes. These lasted from 1896 to the 1950s. Haggard (Author of King Solomon's Mines) was a pioneer in this genre as well.
The genre was filled with Westerns and mysteries. Tarzan was created in 1912 as a serial which went for 22 novels. Then, The Shadow appeared on the radio show Detective Story Hour as the narrator. The publishers soon began receiving requests for The Shadow's stories of his own. Quickly, he earned his own magazine. (Some of his radio episodes can be found here at a link I'll post at the end). The Shadow magazine was published for 325 issues, the longest running pulp magazine ever. Then came the space opera hero Buck Rogers. Doc Savage soon followed. Doc Savage, the Superman precursor, and one of the biggest heroes. The Shadow was dark, mysterious, and dangerous, much like Batman. The Shadow is the archetype of dark, night-walking vigilantes. Doc Savage, on the other hand, was strong, heroic looking, extremely strong, extremely smart, and dedicated to crime-fighting. He acted with the approval of the government, and rarely killed. He's the archtype for Superman.
There were other big ones of course; the Phantom Detective, Operator # 5, G-8 and his Flying Aces, John Carter (Of Mars, yes), Zorro, and the Black Bat. Eventually, they were fazed out by superheroes.
Superheroes really kicked off in the 30s and have lasted in some way, since. Batman and Superman kicked the genre off and it's grown ever since. It deserves its own article.
James Bond novels were written about now. The super-spy. Excellent at disposing of insane enemies. Excellent at driving. Ladies man, of course. He's also a transition between the optimistic pulps and darker side of the Vietnam and Post-Vietnam era. He is a gentleman, (Martinis shaken, not stirred) yet ruthless.
Then came the 60s; flower-power, anti-war, and nuclear fear. Action/adventure took a darker edge with Aggressor novels. They were dark, and violent.
Then the 70s was another transition, to the 80s. Star Wars came out in 1977, a beacon of hope in what I've heard, for SF, was pretty bleak. The 80s were different. Comics, and Batman in particular, were dark and violent. Indiana Jones and the Raiders of the Lost Ark came out in this decade. TV shows included The A-Team, Michael Knight, and other somewhat cheesy action shows. It's also brought about the variety in modern action/adventure.
Since then, we've diversified. The original pulp heroes have gone out of print and new writers have picked them up. Writers inspired by those and Indy began creating their own heroes and stories. Things can be pulpy and optimistic, dark and gritty, or somewhere in between.
The Shadow radio episodes