This paper outlines the research activities conducted by ONERA, focusing on the impact of hydrogen use on the design of transport aircraft. Specifically, the paper presents the research efforts made to enable the study of hydrogen aircraft, understand the impact of hydrogen on the overall design of the aircraft and characterise the performance of a hydrogen fuelled transport aircraft. The study presented in this paper focuses on Small to Medium Range aircraft (SMR) and on two aircraft configurations: a traditional Tube and Wing (T&W) aircraft and a more advanced Blended Wing Body (BWB). The intermediate results suggest that hydrogen has a major impact on the masses and energy efficiency of the aircraft. These impacts depend on the gravimetric index of the hydrogen tank at the first order. Furthermore, the impacts are especially high for T&W aircraft, while the BWB concept seems to be less sensitive to the change of fuel and a better candidate for the use of hydrogen fuel.