The study of atmospheric boundary layer (ABL) flow over smooth hills is an important topic of research with several engineering applications. We study the ABL flow over smooth two-dimensional hills using large-eddy simulations. The numerical framework is based on a high-order Cartesian solver along with an immersed boundary method to represent the non-flat bottom boundary due to the hills. The framework is validated by comparing to previously reported wind-tunnel experiments. We present mean velocities and turbulence intensities of the streamwise velocity for hills of different slopes. Our results show that increasing the hill slope leads to significantly modified flow features as compared to smooth hills. Particularly, flow reversal and separation are not observed for the least steep hill studied here and increasingly large regions of recirculation with large turbulence intensities are observed for steep hills.