To meet increasing data demands, service providers are considering the use of Unmanned aerial vehicles (UAVs) for delivering connectivity as aerial base stations (BSs). UAVs are especially important to provide connectivity in case of disasters and accidents which may cripple completely the existing terrestrial networks. However, in order to maintain the communication of UAVs with the core network, it is essential to provide them with wireless backhaul connection to terrestrial BSs. In this work, we use stochastic geometry to study the impact of millimeter-wave (mmWave) backhauling of UAVs in a hybrid aerial-terrestrial cellular network where the UAVs are added to assist terrestrial BSs in delivering reliable service to users (UEs). In the proposed model, the UE can associate to either a terrestrial BS or a UAV connected to a BS to get backhaul support. The performance of the proposed model is evaluated in terms of coverage probability and validated against intensive simulations. The obtained results unveil that the quality of the UAVs' mmWave backhaul link has a significant impact on the UE 's experience and the deployment of UAVs must be adjusted accordingly.