JetZero, an aerospace startup focused on fuel-efficient jets, will build and fly a sleek new prototype aircraft that could one day join the Air Force’s mobility fleet, the service announced Wednesday. The Z-5 airframe — a blended wing body design with an oblong fuselage and long, skinny wings that looks more like a B-2 Spirit bomber than it does a Boeing 747 — was chosen over one other competitor in a yearlong contest led by the Pentagon’s Defense Innovation Unit. JetZero will ready a full-