Here is a relevant portion from Roddenberry's story outline, dated May 8, 1968:
As we’ll continue developing, Janice Lisette has become more bitter with each passing year over the fact she was born with what she considers disadvantages of a female body. Then suddenly on a minor expedition of a planet, she has discovered the answer -- a device which the ancients of this dead world once used to exchange the consciousness of two bodies. Very likely the device was conceived as a form of immortality whereby old bodies could be exchanged for young, new bodies. An incredible answer to every longing Janice has ever had -- an opportunity to place herself in not only the strong masculine form of a man she always envied, but also at the same time to exchange her life as an unimportant, middle-aged female scientist for the exciting life and prestige of a Starship Captain.
(Helpful to our series, this situation allows us to emphasize as in few other stories that Kirk is one of an elite group -- one of only a few in the entire galaxy holding this rank. As Kirk, Janice will take over one of the most successful of such careers with promotions and futures even more exciting than the present.)
Although Roddenberry's story is rampant with sexist assumptions and pomposity over his work, I do not believe the idea that woman cannot be Starship Captains is ever explicitly stated in the outline. That line must be Arthur Singer's.