Start of the boom
The April–May 1939 issue of Marvel Science Stories
; artwork by Norman Saunders
As Campbell began to hit his stride as editor of Astounding, the first boom in science fiction magazine publishing took off, starting with Marvel Science Stories' appearance in 1938. This was an attempt by publishers Abraham and Martin Goodman to expand their list of titles into science fiction and fantasy. They were known for "weird menace" magazines, a genre incorporating "sex and sadism" into story lines that placed women in danger, usually because of a threat that appeared to be supernatural but was ultimately revealed to be the work of a human villain. For Marvel Science Stories, the Goodmans asked their authors to include more sex in their stories than was usual in the sf field; reader reaction was strongly negative to the spicier stories, but the Goodmans kept the magazine going until early 1941, and eventually revived it in 1950 for a few more issues when another sf magazine boom began. A companion magazine, Dynamic Science Stories, appeared in February 1939; it was intended to carry longer stories but only lasted two issues.
Of more importance to the science fiction and fantasy genres were several other magazines that debuted in 1939. The first to appear, from Standard Magazines, were Startling Stories and Strange Stories, launched in January and February respectively as companions to Thrilling Wonder Stories; Mort Weisinger edited all three. Startling included a lead novel and a "Hall of Fame" reprint section in every issue; the latter was possible because the publisher, Standard Magazines, owned the back catalog of Wonder Stories. At that time no other sf pulp was including novels; readers approved, and Startling quickly became one of the most popular sf magazines. Strange Stories was launched as a direct competitor to Weird Tales, and the first issue featured many of Weird Tales' most popular authors, including Robert Bloch, August Derleth, Henry Kuttner and Manly Wade Wellman. Bloch, Derleth, and Kuttner were all frequent contributors over the magazine's life, but Ashley regards it as a poor imitation of Weird Tales, fewer of its stories having been anthologized since. At the end of 1938 Weird Tales' owner, B. Cornelius, sold his interest in the magazine to William J. Delaney, the publisher of Short Stories, a general fiction magazine. Delaney's offices were in New York, and Weird Tales' editor, Farnsworth Wright, moved there from Chicago. Delaney made several changes to page count and frequency to try to increase circulation, but none were successful. While this took place, another competitor to Weird Tales was launched, this time by Street & Smith. The new magazine, titled Unknown, was a companion to Astounding and was also edited by John W. Campbell. Campbell's declared policy for the magazine was to "offer fantasy of a quality so far different from that which has appeared in the past as to change your entire understanding of the term"; the goal was to bring "the science fiction rationale to fantasy", in Ashley's words. Unknown's first issue was dated March 1939, with L. Ron Hubbard and L. Sprague de Camp soon among the most frequent contributors.
Also in March 1939, a new publisher entered the field: Louis Silberkleit, who had once worked for Gernsback, was the owner of the Blue Ribbon Magazines imprint; he launched Science Fiction, following up with Future Fiction in November that year. Both were edited by Charles Hornig, who had edited Wonder Stories for Gernsback. Silberkleit gave Hornig a very limited budget, so he rarely saw submissions from established writers unless they had already been rejected by the better-paying markets. The result was mediocre fiction. Silberkleit was also slow to pay, which was an additional discouragement to authors deciding where to send their work. In May Ziff-Davis, the publishers of Amazing Stories, joined the fray, with Fantastic Adventures, also edited by Raymond Palmer. As with Amazing, Palmer focused on entertainment, rather than trying to break new ground. Fantastic Adventures was not positioned as a rival to Weird Tales or Unknown, but focused instead on other-worldly adventures in the style of Edgar Rice Burroughs, and soon developed a reputation for whimsical fantasy as well.
The original pulp publisher, the Munsey Company, still had no dedicated science fiction or fantasy magazine by this time, but frequently published stories in Argosy and All-Story which were clearly within the genre. At the end of 1939 Munsey launched Famous Fantastic Mysteries as a vehicle to reprint these older stories. The editor, Mary Gnaedinger, choose Merritt's "The Moon Pool" and Ray Cummings' "The Girl in the Golden Atom" for the first issue, dated September/October; both titles were likely to attract readers. Gnaedinger engaged Virgil Finlay to do interior illustrations for the second issue, and the magazine was soon successful enough to switch from bimonthly to monthly publication.
The ninth and last new science fiction or fantasy magazine to appear with a 1939 cover date was Planet Stories, its first issue dated December. The publisher was Fiction House, which had been a successful pulp publisher in the 1920s, though the Depression temporarily closed its doors. After a relaunch in 1934 Fiction House specialized in detective and romance magazines, and Planet was published by its Love Romances, Inc. imprint. The editor, Malcolm Reiss, targeted younger readers, and focused solely on interplanetary fiction, though initially the stories were so weak that Ashley speculates Reiss was forced to start the magazine without enough time to acquire worthwhile material.