The main goal of this study is the development of an aggregate air itinerary share model, estimated at the city-pair level, for the US air transportation system. This route demand assignment model is part of a new modeling approach that has as its ultimate output the prediction of detailed traffic information for the US air transportation system. In this approach, city-pair demand generation, route demand assignment, and air traffic estimation are completed in 3 different stages within a single framework. Aiming to fully develop the overall model, in this paper we focus on estimating the 2 nd stage, the air itinerary market share model. In order to achieve this, we apply multinomial logit models for itinerary share estimation. The models are developed at an aggregate level using open-source booking data for a large group of city-pairs within the US Air Transportation System. Although there is a growing trend in the use of discrete choice models in the aviation industry, existing airitinerary share models are mostly focused on supporting carrier decision-making. Consequently, those studies define itineraries at a more disaggregate level, using variables describing airlines and time preferences. In this study, we define itineraries at a more aggregate level, i.e., as a combination of flight segments between an origin and destination, without further insight into service preferences. Although results show some potential for this approach, there are challenges associated with prediction performance and computational intensity. The air itinerary share results obtained will be used to predict air traffic levels in future work.