What's the first story featuring mass mindwipe (amnesia) of all humanity? (cf. A Wind Named Amnesia)
FuzzyBoots's link to a relevant Science Fiction Archive article turned up another tidbit: Rebirth: When Everyone Forgot by Thomas Calvert McClary. Serialized in Astounding Stories in February and March 1934, it was later published in book form in 1944.
Mentally, you moderns are children of Goddard's madness.
What good are brains and research and scientific progress if they accomplish nothing more than to swell their own importance and create wealth for a few and misery for millions?
Goddard, a brilliant scientist disgusted with the wars and inequalities of the world invents a "migroid ray" gun that suppresses the "blue rays" emanating from every human brain, imposing total amnesia on the human race, including Goddard, except one person (the narrator) who was shot up into space after he lost a bet.
Goddard's idea was that people of superior intelligence would rebuild the world into a better place, but the results were far from Goddard's wish.
I've really only skimmed over parts of this story. A Goodreads review of the story indicates that it is sexist, racist, elitist, and probably several other-ists thrown in to boot, and probably not worth the trouble reading. Goddard's viewpoint is called out in the much better story "The Marching Morons" by Cyril Kornbluth.
Thanks to FuzzyBoots for graciously allowing me to post an answer based on their research.
As per my comment from The Encyclopedia of Science Fiction entry on amnesia, the earliest example I've found is Robert Silverberg's "How It Was When the Past Went Away"
“A loony with a bunch of nifty drugs puts amnesifacients in San Francisco’s water supply,” reads the story’s description on the “Quasi-Official” Silverberg website, “The city starts to fall apart as people forget who they are, who they’re married to, where they work. The drug effects everyone a little differently, with different memories lost to differing degrees. The story is told in true disaster-movie tradition, with a multitude of characters from different walks of life followed through the crisis. There’s the artist so in debt he can’t afford the tools of his trade, but doesn’t remember his monetary problems or that his wife left him. There’s the crooked stock broker with millions worth of illicit transactions kept only in his head. There’s the grieving man who lost his family in a plane crash. There’s the nightclub performer whose act is that he remembers everything.”
However, that's partial amnesia, and it looks like it's largely in San Francisco.
"Sketches in the Ruins of My Mind" by Philip José Farmer is a bit closer except that it's a progressive form of amnesia that strips four days of memories every night.