A theoretical model of the flow in 3-D axisymmetric ducts with high subsonic intake Mach numbers is developed. The model is based on the iteration between the axisymmetric potential core flow and a proven cannpressible turbulent boundary layer method based on the entrainment principle. The solution of the core flow partial differential equations is achieved through expanding both the stream function and the density function as power series of the duct radius thus reducing them to a series of linear simultaneous ordinary differential equations. The boundary layer method is capable of handling excessively disturbed inlet flows, by accounting for upstream history effects on the boundary layer development. A single integral parameter is used to describe the inlet turbulence structure. The model is verified using experimental data available in the literature, and the verification proved its success in predicting the pressure recovery of the intake system and its sensitivity to variations in inlet flow and geometrical parameters.