It's a Wonderful Life tells its story partly through an extended flashback and partly in real time. It starts at the point when George is contemplating suicide and then goes back to his early life, as the senior angel Joseph tells Clarence the story of George's life. We see George saving Harry, preventing Mr. Gower from accidentally poisoning that kid, defusing the run on the Building and Loan, and so on. Joseph also shows us moments in the life of the townspeople, explaining what many of the characters did during World War II.
When we get to the point when George is thinking of killing himself, Joseph's narration stops, and we see the movie unfolding in present time when Clarence arrives. We then enter an alternate timeline with George—the world in which he was never born—but one that's still taking place on Christmas Eve of 1945. Finally, we're back to the world-with-George timeline and roll straight through chronologically until the end. Throughout, the movie retains a focus on George, though you wouldn't exactly call it "first person" since we're not seeing things from George's perspective, but rather from the angels'.