A new hierarchical software architecture is proposed to improve the safety and reliability of a safety-critical drone system from the perspective of its source code. The proposed architecture uses formal verifi cation methods to ensure that the implementation of each module satisfi es its expected design specifi cation, so that it prevents a drone from crashing due to unexpected software failures. This study builds on top of a formally verifi ed operating system keel, certifi ed kit operating system (CertiKOS). Since device drivers are considered the most important parts affecting the safety of the drone system, we focus mainly on verifying bus drivers such as the serial peripheral interface and the inter-integrated circuit drivers in a drone system using a rigorous formal verifi cation method. Experiments have been carried out to demonstrate the improvement in reliability in case of device anomalies.