The lovely Superman website, Superman Through the Ages has done a wonderful job on this story, going so far as to actually presenting a recreation of the story!!
You see, in 1940, Jerry Siegel and Joe Shuster (well, at least Joe Shuster's art studio of Joe Shuster himself, along with Paul Cassidy, Wayne Boring, and Leo Nowak) worked together on a story that was to appear in early 1941's Superman #8.
In the story, Superman's identity is revealed to Lois Lane when they are trapped in a cave-in together (and Clark can't think of a way out of there without revealing the truth to Lois).
Lois is irked at Clark for this, but in the end, she ultimately decides it is best to keep his secret and aide him in the future (like coming up with excuses, etc.). This was going to be the status quo of the book from then on, but ultimately,
However, the higher-ups killed the story, and the drawn art was never heard of for about two decades, until Jim Steranko published four pages of the artwork in his 1970 book, History of Comics, Volume 1. They were grainy, and the story was illegible.
It wasn't until eighteen year laters, when DC staffer Mark Waid found Jerry Siegel's original script that the story was finally revealed!