The study will evaluate the safety and efficacy of delandistrogene moxeparvovec gene transfer therapy in non-ambulatory and ambulatory males with DMD. This is a randomized, double-blind, placebo-controlled 2-part study. Participants will be in the study for approximately 128 weeks. All participants will have the opportunity to receive intravenous (IV) delandistrogene moxeparvovec in either Part 1 or Part 2.