"Death Ray" is a phrase most commonly associated with the Golden Age of science fiction, before the Second World War. But what people forget is that in those days, death rays were perfectly valid speculative science, and by no means the sole province of science fiction. Here are a few examples of what I mean -- three factual (or allegedly factual) and two fictional (but not science-fictional).
 A clue to the Air Ministry's interest in the subject may be contained in File AIR 5/179 at the UK Public Records Office, which includes a memorandum dated 4 April 1924 claiming (erroneously, as it turns out) that Germany was in possession of "an apparatus from which rays (or electric waves) can be projected to a height causing aeroplane engines to break down".
 In 1935 the Air Ministry asked a scientist named Robert Watson-Watt to investigate "the practicability of proposals of the type colloquially called death ray". Watson-Watt carried out some experiments, which showed that it was impossible to transmit sufficient energy to destroy an aircraft in flight. However, he discovered in passing that the energy reflected back could be picked up again, allowing the distant aircraft to be detected. This discovery led within a few years to the invention of radar... which was to have almost as great an effect on aerial warfare as a death ray would have!
 Because of the popular interest in death rays during the 1920s and 30s, they turned up in mainstream fiction as well as science fiction. Perhaps the last place you'd expect to find a death ray is in one of Agatha Christie's novels about the master detective Hercule Poirot, but there is one in The Big Four (published in book form in 1927, but serialized in a magazine a few years earlier). The eponymous villains ("the Big Four") have acquired "some powerful wireless installation -- a concentration of wireless energy far beyond anything so far attempted, and capable of focusing a beam of great intensity upon some given spot"... and are using it to destroy ships and aircraft. As a novel, The Big Four is well below AC's usual standard (it's the closest she ever got to writing pulp fiction)... but it isn't science fiction. The death ray (and also atomic power) feature in the novel simply because they were big topics of speculation at the time.
Q Planes from 1939, which starred Laurence Olivier (and you can't get more mainstream than that!). The villains use their ray (illustrated on the left) to bring down experimental planes which are testing a new kind of engine supercharger. What is amusing about this film, in retrospect, is that the invention everyone is after (the McGuffin, in other words) is the new supercharger and not the death ray!