Complex sentence: Producer Lucien Hubbard hired director Wellman because of his World War I aviator experience, the only director in Hollywood at the time who had combat pilot experience, although actors Richard Arlen and John Monk Saunders had also served in World War I as military aviators.
Split into simpler sentences: Producer Lucien Hubbard hired director Wellman because of his World War I aviator experience, the only director in Hollywood at the time who had combat pilot experience. Arlen, Wellman, and John Monk Saunders had all served in World War I as military aviators.