PRONAOS was born as a project in the mid eighties. The Far Infrared Space Telescope (now Herschel), a Cornerstone of the horizon 2000 plan of ESA, was the clear future of submillimetre astronomy, after several balloon experiments and the tremendous success of the IRAS satellite in the neighbour field of infrared astronomy. Guy Serra and other French scientists proposed to build a learning process in this new field around the PRONAOS project, a technical and scientific precursor to future submm space missions. This project was fully supported by CNES and by CNRS (IAS and CESR at the first place). PRONAOS (Projet National d'AstronOmie Submillimétrique) was based on the use of a two meter telescope aboard a stratospheric balloon-borne gondola. The size and the complexity of the instrument made PRONAOS a very unusual balloon-borne payload. Along the 15 years of the project, and three flights, a lot of learning processes took place. The main goals of this enterprise were reached. New and sometimes surprising scientific results came out from the observations. Teams learned critical scientific and technical lessons from this pioneering experiment. Some contributions to Herschel benefit directly from this experience, and the project Planck, a satellite of ESA, was born inside the PRONAOS team. This project was also a major professional and human experience for young (and also mature) scientists.