Being affected throughout their lifetime by a strong mass-loss due to radiative winds, fast rotation, and a high binarity rate, massive stars pose several challenges to the understanding of their observational properties and evolution. The aim of our project is to improve our knowledge of these objects by coupling state-of-the-art two-dimensional simulations of stellar interiors including rotation, pulsations, and mass-loss, with radiative transfer models of their atmospheres and environment, in order to compare the resultant predictions with observations at the highest spectral and angular resolutions. This unique combination of fundamental modelling, radiative transfer, and confrontation with observations will enable us to improve the models of massive stars, as well as our understanding of the underlying physics. Our project relies on the successes of the ESTER code (Toulouse), which models rapidly rotating early-type stars in two-dimensions, and of the TOP pulsation code (Paris-Meudon) for carrying out asteroseismic inferences. From the combination of these codes with radiative transfer models for the atmosphere and the circumstellar environment (winds and disks, which can be composed of gas and dust), we will create a complete and physically consistent modelling tool of massive stars (typically with masses between 4 and 20 solar masses). This physical model will allow us to simulate polychromatic images from which several observables can be computed, in particular those for long baseline optical/infrared spectro-interferometry. We will use this model to compute and deliver to the community, improved grids of models of typical massive stars (interiors, atmospheres, and extended environment). Tools to use these model grids for the analysis of spectro-interferometric data (innovative imaging and model fitting methods) will be developed/adapted for the project and also provided to the community. In addition to these models and dedicated analysis tools, the success of our project for a deep physical study of massive stars relies on the unique expertise in the development and exploitation of spectro-interferometric instruments in Nice, in particular MATISSE, the new mid-infrared instrument of the Very Large Telescope Interferometer (VLTI) array at ESO-Paranal, and SPICA, the upcoming visible beam-combiner for the Center for High Angular Resolution Astronomy (CHARA) array at Mount Wilson Observatory. From our physical analysis of many existing and near-to-come spectro-interferometric data, we will be able to constrain many key parameters defining the central star (mass, temperature, age, rotation rate...) and its environment (mass-loss, and wind and disk structures, i.e. density and temperature law, chemistry, dynamics) of about 200 stars from our survey list. In particular, we will obtain a detailed view (including models and reconstructed images) of about 25 primary targets, which will constitute a unique set of landmark massive stars, spanning different types (e.g. classical Be, B[e], fast rotators, beta Cephei stars, slowly rotating B stars, supergiants). The results of this unprecedented study will also provide invaluable information for a global, unified understanding of the structure and evolution of massive stars.