Brewster McCloud (1970) starring Bud Cort (of "Harold and Maude" fame), Shelley Duvall, Sally Kellerman, Michael Murphy, William Windom, Jennifer Salt
IMDb synopsis: Brewster is an owlish, intellectual boy who lives in a fallout shelter of the Houston Astrodome. He has a dream: to take flight within the confines of the stadium. Brewster tells those he trusts of his dream, but displays a unique way of treating others who do not fit within his plans. When the fateful day arrives, and he enters the dome with his fanciful construction of bird wings, Brewster is surrounded by the police. Will he be caught before he attempts to fly?