Heinlein's first novel, For Us, the Living was written in 1938 though not published until many years later, decades later in fact. It had a rich back-story, or the beginnings of one at any rate and even though the novel itself got shelved, the back story started to get used for a lot of short stories.
In 1939 he was working on a novella called Vine and Fig Tree which fitted into the: For Us, the Living future history and became If This Goes On;  it concerned how how the United States recovered from a religious dictatorship. His intention was to interconnect the stories as he wrote them, linking names and incidents in one story to others. This quickly became unwieldy (there were no Wikis then, after all) and he had to work through a lot of notes each time he wanted to refer to one story from another. He decided to use the same method as the author Sinclair Lewis did to organize the stories of "Zenith" in the fictional midwestern state of "Winnemac".
William Paterson quotes Heinlein as saying the following in the Science Fiction Writers of America bulletin in 1975:
I took an old navigation chart, about 3 x 4 feet, turned it over, made the time scale vertical, then set up five columns: stories, characters, technical data, sociological, remarks.Heinlein mentioned this (by then much edited and very messy) chart in passing to his editor John W. Campbell, unsurprisingly Campbell wanted to see it; it was published in the May 1941 issue of Astounding along with two Heinlein stories, Blowups Happen and Universe.
The Chart Edit
This is the chart as it was published in 1941.
Timeline image from http://www.baenebooks.com/chapters/1439133417/143913341701.jpg, included for historical and documentation purposes only. No infringement of any copyright intended.
Timeline Stories That Didn't Get WrittenEdit
Titles in () above, such as (Word Edgewise) were planned but never written, for one reason or another.
- Word Edgewise
- Fire Below
- The Sound Of His Wings
- The Stone Pillow
- Da Capo - planned to be written in the 1950s, it actually became the concluding section of Time Enough for Love in 1973.
These unwritten stories have tantalised more than one Heinlein fan. Not least, one Laurence M. Janifer who wrote The Counterfeit Heinlein: A Gerald Knave Science Fiction Adventure which includes portions of a lost manuscript of The Stone Pillow, rediscovered when - but I'll let you read it for yourself, it's on Google Play.