From late May 1938 through winter of 1939 Steinbeck was writing The Grapes of Wrath.
He wrote this novel for about seven months. However, this novel was a final accumulation of his previous published articles and failed books on the topic of migrants' condition.
Initially, Steinbeck was commissioned by The San Francisco News to write a seven-part series of newspaper articles from October 5 to 12, 1936, titled The Harvest Gypsies. His articles were full of alarming facts about migrants’ lives (illness, incapacitation, death). A year later in late 1937, he attempted to write as he called a “rather long novel” called The Oklahomans. He did not finish the novel.
He continued to write about his vivid investigation of migrants’ lives from February to May 1938 that later became “L’Affaire Lettuceberg. After he finished the first draft of the book, which was a little over seventy thousand words, he destroyed it. He said to his main literary agent, "My father would have called it a smart-aleck book. It was full of tricks to make people ridiculous. If I can’t do better I have slipped badly (…) It is sloppily written because I never cared about it. I had got smart and cocky you see (…) A book must be a life that lives all of itself and this one doesn’t do that.”
Then after everything that he'd done, he started to draft The Grapes of Wrath.
Reference:
Steinbeck, J., & DeMott, R. J. (1989). Working days: The journals of the Grapes of wrath, 1938-1941. New York: Viking.