The first significant "original" anthology -- containing stories that had never been published before -- was New Tales of Space and Time in 1951 from Raymond J. Healy, who had co-edited Adventures in Time and Space five years earlier. (J. Francis McComas, meanwhile, had become co-editor of The Magazine of Fantasy & Science Fiction.) First published in hardcover and then in three paperback editions, the book included several significant stories, notably Bradbury's "Here There Be Tygers," Kris Neville's "Bettyann," and Anthony Boucher's (later SF Hall of Fame story) "The Quest for Saint Aquin."
The book introduction by Anthony Boucher makes explicit the key motivation for assembling an original anthology: that so many anthologies of reprinted stories were being published that the field was "getting into a pretty bad way.... [T]here simply are not that many magazine stories worth reprinting." And readers would be dismayed to encounter the same stories in one anthology after another. "The anthology of unpublished stories is an answer to this situation."
Healy followed with 9 Tales of Space and Time in 1954. Its contents include a second "Bettyann" story by Kris Neville.
The image shows a cover images of the hardcover first editions, and a photo of the first printing of the Pocket Books paperback of the first book.