## 1 Introduction  
The decarbonization of air transport by 2050 is not a negotiable target and, in order to pursue it, breakthrough technological solutions should be adopted, such as the development of unconventional airframes, such as truss braced wings [1, 2], the blended wing body [3, 4, 5, 6], and the box-wing (BW) [7, 8, 9, 10], and the integration of innovative propulsion systems, such as hybrid-electric [11, 12, 13] and hydrogen-based [14, 15]. On one hand, unconventional airframes may allow the increase of the lift-to-drag ratio and/or the structural efficiency with respect to state-of-the-art [16, 17]: on the other hand, the adoption of innovative hybrid powertrains may enable reductions in fuel consumption and greenhouse gas emissions [18, 19, 20]. Battery-powered aircraft are suitable for regional aircraft [21, 22, 23], whereas hydrogen-powered aircraft seem more relevant to medium- and long-range aircraft [24, 25]. Electric aircraft are under deep investigation as, ideally, this specific technology can totally suppress flight-related CO\({}_{2}\) emissions and reduce noise during close-to-ground operations [26]. Nowadays, the largest barrier to aircraft electrification is the low specific energy density of the batteries: for the current Li-ion batteries, this important index is about 250 Wh/kg at the cell level [27, 28], and it is not sufficient to provide enough energy for transport aircraft. Power electronics, electric motors, and batteries are technologically moving forward, and the related technology forecast allows for optimism about the use of electric propulsion on regional transport aircraft [29; 30]. Currently, several works are available in the literature reporting the environmental benefits coming from the adoption of different airframes or different propulsion systems, but the coupling of these two technology elements seems to represent the proper step forward to achieve major improvements. In this regard, the authors' recent review [21] discusses the coupling between the BW airframe concept and the hybrid-electric propulsion system in a regional class aircraft and shows the potential of joining these technology bricks to cut greenhouse gas emissions. Following these encouraging results, this paper aims to assess a comprehensive performance comparison between a BW and a classical tube-and-wing (TW) hybrid-electric aircraft. In fact, as highlighted in previous works [31; 32; 33], the BW airframe may represent a more efficient lifting architecture that could introduce different gains to aircraft performance, namely: _(i)_ the optimal design of the BW leads to the minimum induced drag with respect to the corresponding monoplane with the same lift and wingspan; _(ii)_ the lifting system is over-constrained to the fuselage, providing the possibility to exploit this feature to achieve structural mass reductions; _(iii)_ the possibility to design the coupling of the lifting system and fuselage to enlarge the available internal volume, hence providing benefits if there is a necessity to store components with large volumes, such as large battery packs; and _(iv)_ a higher generated lift for the same wingspan as TW, without any penalization in the overall lift-to-drag ratio, hence allowing to efficiently transport larger weights and to comply with current airport apron constraints [7]. The focus of this work is, therefore, on the comparison between a BW and a TW configuration equipped with a hybrid-electric powertrain. In this context, to fairly compare the two configurations, the same assumptions on the propulsion system have been considered, namely: _(i)_ a parallel architecture is selected for the hybrid-electric powertrain; and _(ii)_ the envisioned entry to service is 2035 so that the electric powertrain components performance have been defined to be compliant with the forecast for such a time horizon.