C.E. Evans wrote:
As far as onscreen material goes, there's no mention of Kirk commanding any other ship prior to the Enterprise.
Incorrect. In the character's bio reprinted in The Making Of Star Trek
it says Kirk commanded a destroyer equivalent class starship. This is reinforced by a reference in WNMHGB when Elizabeth Dehner metioning Kirk having asked for Mitchell aboard his first command.
That's a nice touch of realism, there, suggesting Kirk had some command experience before being posted to one of Starfleet's frontline ships.