A method of forming a monolithic aircraft structure having multiple aerodynamic surfaces includes forming a body component to have a body skin defining a body skin outer surface, and a body side wall integrally formed with the body skin and defining a body mating surface, the body skin outer surface providing a first aerodynamic surface. A cover component is formed to have a cover mating surface and a cover outer surface opposite the cover mating surface, the cover outer surface defining a second aerodynamic surface. The body component is positioned relative to the cover component so that the body mating surface engages the cover mating surface. At least portions of the cover mating surface are friction stir welded to the body mating surface to form friction stir welded joints between the body component and the cover component.