In skimming flow on a stepped spillway, the upstream flow motion is nonaerated and a turbulent boundary layer develops until the outer edge of the boundary layer interacts with the free surface; that is, at the inception point of air entrainment. Herein, new experiments were performed in the developing flow region on a large 1V:1H stepped spillway model with step height h=0.10m. The flow properties in the developing flow region were carefully documented. In the developing boundary layer, the velocity distributions followed a 1/4.5th power law at the step edges. Detailed velocity and pressure measurements showed some rapid flow redistribution between step edges and above step cavities. The application of the momentum integral equation indicated an average friction factor of 0.18, which is close to the observed air-water-flow friction factor of 0.23, suggesting that the spatially averaged dimensionless shear stress was comparable in the developing flow and fully aerated flow regions.