The conceptual design of a subsonic, environmentally efficient, twin-aisle commercial airliner is presented (previously presented by Yutko et al ). The D8 double-bubble aircraft - named for its complex, non-round fuselage shape - originated from NASA's N+3 Phase I study in which participants designed efficient commercial aircraft for market entry in the 2035 timeframe. Previous computational and experimental work has established the potential of the D8 configuration to significantly reduce environmental impact of aviation. However, the integration of engines and airframe, as well as the non-round fuselage with central support element, pose many challenges for the airframe designer. In this paper, the conceptual design of a D8 aircraft is presented: the aircraft is designed to comply with FAR 25 requirements and air transportation system constraints; airframe structural solutions for the unique configuration challenges of the D8 are presented; aircraft weight and balance and airline operations are analyzed; aerodynamic lofting and performance is accomplished with CFD; and performance of the boundary layer ingesting (BLI) propulsion system is investigated. Results are presented for two D8 concepts. First, a concept for a D8 with an entry-into-service (EIS) of 2016 was designed that utilizes current engine technology and existing composite manufacturing techniques, and it is demonstrated that such an aircraft is capable of saving between 25-30% fuel when compared with a Boeing 737-800. Second, a concept for a vision system with EIS prior to 2035 that is capable of meeting NASA's mid-term environmental goals was designed. An air transportation system analysis was completed using these aircraft and it is shown that the D8 configuration has the potential to reduce narrowbody system fuel consumption by 52% and significantly reduce community noise impacts based on an analysis of the top 20 United States airports.