A hybrid propulsion system for aircraft application represents a new paradigm for propulsion systems designers. Several new problems appear, and classical design methods must be challenged. Indeed, addition of new systems and new components inside the aircraft architecture imply the need for a multidisciplinary study, in order to support design and integration. For example, addition of a battery pack to a propulsion system requires functional analysis, performances estimation, energy management and operational strategies. More than these aspects, related to design, integration is also challenged: where to locate the battery pack, how to manage thermal effects, how to manage electromagnetic effects... To address these two parts, design and integration, early in the design, we propose to build a model of intention. This article is dedicated to the concept of model of intention in a complex system design project. This concept will be explained, justified, developed and tested in order to allow reader to have clear vision of what represents the model of intention.