In their book "How Underdog Was Born...", W. Watts Biggers and Chad Strover reveal that seeing Wally Cox's performance in this movie inspired them to ask him to voice their newly created character, Underdog.
According to a Henry Fonda interview seen on the DVD version of this movie, many locals of Jackson Hole, Wyoming were used as extras for scenes in the movie, such as the graduation of Clayboy.
In the scene where Clay is milking the cow, "Chance," she is first seen without a halter, then with one in the next shot, then again moments later without it.