What are the similarities between the first and last dream in "The Secret Life of Walter Mitty"?
I think the similarities seem to lie more in the kind of heroic display of character that Walter Mitty is able to make in both daydreams rather than the actual situation. However, we can perhaps find some similarity in the way that Walter Mitty always chooses impossible situations for his heroic persona to face in his daydreams. In the first daydream, Walter Mitty is the commander of a plane who has to go through a massive storm. His men are terrified that the aircraft will be destroyed in the storm, but Walter Mitty insists they go on through and we are shown the admiration with which he is regarded by his men because of his bravery and leadership. The last daydream features Walter Mitty facing the firing squad and scornfully rejecting a handkerchief to blindfold his eyes. Notice the way he is described at the end:
Then, with that faint, fleeting smile playing about his lips, he faced the firing squad; erect and motionless, proud and disdainful, Walter Mitty the Undefeated, inscrutable to the last.
Both daydreams represent escapes from an overbearing wife, quite ironically in the last one, as it presents death as preferable to being with her, but at the same time they show a tragic inability to confront Walter Mitty's own problems by creating this do-or-die situations that, in his daydreams, he can resolve.