Great question!

As it has been said time and time again, all art is derivative of something else, and Flash Gordon is no exception.

The Flash Gordon serials originated as a Sunday comic strip in 1934 under King Features Syndicate. The character was designed, primarily, to compete with another popular science fiction hero of the time, Buck Rogers, who had been massively successful.

Like what George Lucas faced when he attempted to obtain the rights to make a Flash Gordon film (and failed, giving us Star Wars), King Features had attempted to get the rights to Edgar Rice Burroughs’ John Carter of Mars and failed, according to biographer Robert Fenton.

Another source of inspiration for the comic was the fairly new (at the time) novel When Worlds Collide, which was adapted by artist Alex Raymond for the first storyline and characters; this storyline was rejected at first, due to a lack of action sequences. Raymond reworked them to fit what King Features wanted and resubmitted them, and they were approved. Raymond and a ghostwriter named Don Moore were paired up (which was - and still is - common in the comics world), and the Flash Gordon comics became a massive success, running until the 1990s and being turned into toys and books... and yes, serials.

The serials, however, were somewhat different in their approach. Whereas Flash Gordon the comic was meant to sell merchandise, Universal (the company in charge of the serial’s production) wanted to market the motion picture to a more adult crowd, and put it in theaters in large cities (or “a-level”). This serial - and its sequel, Flash Gordon Conquers the Universe - were the main inspirations for George Lucas when he made Star Wars.

In conclusion, Flash Gordon was inspired directly by Buck Rogers, but also took influence from other media of its time. Its creation, like the films it influenced, was also caused by the inability to get the rights to an already existing property.