Pulp magazines evolved in the late 19th century from earlier fiction magazines. Rival publishers learned that fat, cheap magazines sold incredibly well. At first, they covered all types of fiction indiscriminately, including those stories called "scientific romances," back when a romance was synonymous with an adventure. Reader feedback made the publishers rethink that policy. People who wanted a certain type of story were growing frustrated by having to skip past hundreds of pages to get to something they liked.

 

Specialty pulps appeared sporadically at first, but by the 1920s most pulps, except for the few top-of-the-line elites, limited themselves to a single subject. What we now call science fiction comes from one man, Hugo Gernsback, the leader in radio and even television magazines and allied electronic subjects. His Amazing Stories created the genre and when he lost control of the magazine his later series of Wonder publications competed with Amazing and Astounding to create the Golden Age of Adjectival Titles.

 

Robots were slow to penetrate science fiction. My database shows fewer than a dozen stories about robots appearing in the 1920s. By the 1930s, though, writers and artists realized the possibilities of the robot menace, a scourge that could substitute for aliens and giant monsters to wreak havoc on American cities and menace the stalwart heroes. No image of what a robot should look like had coalesced in the popular mind, so artists could create wild and mechanically implausible monsters to freeze buyers' blood. I can't explain the fascination for tentacles that so many had.

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I've tried to pull covers from many different magazines and artists with no attempt to be comprehensive. Information about the covers is from ISFDB.com.