Tarzan had to find a son, because censors would not allow Jane to become pregnant since she and Tarzan were not legally married.
Maureen O'Sullivan had indicated she was tired of the Jane role, and so, Boy was brought in as a substitute interest. In the original screenplay, Jane is hit in the back by a spear while rescuing Boy, and she dies soon after Tarzan finds her. Edgar Rice Burroughs wrote, stating he regretted the killing of Jane in the screenplay, feared that the ending would have a bad result at the box-office, and wrote an alternate ending. The original ending was shot and previewed; but since audiences disapproved of Jane's death, the alternate ending was used, giving Jane a reprieve.
Johnny Sheffield claims that he was handpicked by Johnny Weissmuller after screening hundreds of applicants for the role of Boy. Weissmuller is also the person who taught Sheffield to swim for the movie.
It is stated that Boy is the heir to the Greystoke title. In the original books, it was Tarzan himself who was the heir.
The broken rifle belonged to Captain Fry, who was in the previous film, Tarzan Escapes (1936). Angry that an animal had been shot, Tarzan took Fry's rifle and smashed it against a tree.