What type of character is Walter Mitty?

Walter Mitty, the main character in "The Secret Life of Walter Mitty," is a daydreamer.  He spends a good deal of his time imagining that he is someone else.  His daydreams all have him as a successful, courageous, heroic individual, who is called in to save the day.

"He imagines himself the hero of his fantasies as a navy pilot commander, doctor, sharpshooter, bomber pilot, and noble victim of a firing squad. Mitty is married to a woman who treats him more like a child than a husband. This is due to his immature tendency to escape into fantasies rather than live in the real world."

In real life, Walter Mitty is a bumbling fool who would rather spend his energy dreaming of things he isn't, rather than make a real change in his life. 

Throughout the story, Walter Mitty changes very little, the only thing that changes are his daydreams.  In his final daydream, he imagines himself facing a firing squad.  Of course this is another expression of his exceptional courage and bravery.  But I always wondered if this daydream didn't mean something more, like maybe he had a secret desire for death to escape his boring, controlled existence under the constant nagging of his wife.

This thought gives some credibility to Mrs. Mitty's concern for Walter Mitty's health.  He clearly suffers from some mental disorder in my view.