Here are three clues:
1. The artist of the famous poster, James Montgomery Flagg, chose this man to be his model because he had “the longest arms, the longest nose, and the bushiest eyebrows.”
2. The artist was one of America's leading illustrators.
3. It was a recruiting poster "designed to instill in the people a positive outlook, a sense of patriotism and confidence."
Answer: "I Want You" by James Montgomery Flagg, 1938.
Rockwell image: https://www.pinterest.com/pin/372391462930600173/
(Norman Rockwell's cropped image, a man walking with his dog in the country, was published in the Saturday Evening Post).
Image of Uncle Sam: https://knowyourmeme.com/memes/uncle-sams-i-want-you-poster