Part of me thinks there was a narrow chance of a woman being elected president in the 40s with Frances Perkins becoming VP and succeeding Roosevelt. She was obviously close to him being one of the few people to last his entire presidency.
Keep in mind that the scenario is asking if a woman could be elected outright. The post-World War II purge of the rosies- and the insistence throughout the war that women's gains were temporary- makes me doubt that she's taken seriously. If she falls into the job like you suggest, she's seen as a caretaker until the next election, which itself would be a testament to the progress made by that first waver generation of suffragist activists.
There are examples from that time period of women who fell into office but were able to campaign and keep it. Hattie Carraway is an example of this. If, for example, Perkins succeeds FDR in 1945 she’s have essentially a full term to get the party behind her and set up for re-election in 1948.
It’s not a likely scenario, but I think it is the earliest a woman could have been elected president. After that I don’t think till the 70s of 80s though.