Harl Vincent's "Terrors Unseen" from the March 1931 Astounding Stories was a curious mix of robot-gone-berserk science fiction and gangster story. "The Barrier", from the September 1934 Amazing Stories was another genre crossover: a spy story placed in an interplanetary setting.
A common plot of spy stories (including the real-life spy story used to frame Alfred Dreyfus) is that of a foreign agent who acquires military secrets from traitors working for the government. Vincent's twist was to have the foreign agent infiltrate a military research facility and steal the secrets directly from the scientist who was discovering them. In other respects, "The Barrier" is much like any other spy story. Although the foreign agents are Martians, they are not particularly alien, looking enough like human beings that the Martian leader could call himself Boris and pose as an Istrian. It was common in the science fiction of the time for beings from other planets in the Solar System to resemble human beings, and even (as in "The Barrier") to form romantic attachments with humans. However, a revolution in science fiction aliens was already under way.
Two months before "The Barrier" appeared, Stanley G. Weinbaum published his first story, "A Martian Odyssey", in the July 1934 issue of Wonder Stories. Weinbaum's Martians were emphatically not human beings; they were completely alien -- so much so, in fact, that the story's human characters weren't always able to understand their actions. During his tragically brief writing career, Weinbaum's alien landscapes in such stories as "Parasite Planet" and "The Mad Moon" would permanently transform the depiction of aliens in science fiction.
Which is not to say that science fiction writers stopped writing about human aliens. Vincent would continue to do so, and later writers such as Leigh Brackett, Ray Bradbury, and H. Beam Piper would continue to populate Mars with human Martians right up until the 1960s. In visual media, the need to use human actors to portray aliens would ensure that human aliens would remain the rule rather than the exception right up to the present day.