A turbofan engine includes an outer fixed structure, the downstream terminal end of which terminates in a terminal plane substantially normal to the longitudinal axis of the engine. The radius R of the internal surface of the outer fixed structure in the terminal plane is a function R() of azimuthal position such that the radius has a constant value R0 within a first azimuthal interval of 180, a value greater than R0 at any azimuthal position within a second and fourth azimuthal intervals and a value less than R0 at any azimuthal position within a third azimuthal interval, the azimuthal intervals forming a total interval of 360 and the fourth interval being contiguous with the first. The engine may be mounted closer to the airframe of an aircraft than a turbofan engine having an outer fixed structure which is axisymmetric in its downstream terminal plane.