The project outlines an R&D programme for the development of critical accelerator technologies related to ion therapy; the programme builds on existing activities and on core CERN competences. Two accelerator designs have been identified by a large hadron therapy community as the most promising options for future ion therapy facilities: a superconducting synchrotron and a high-frequency linac. Instead of fully developing the two alternative designs, the present project aims at concentrating CERN funding only on the development of few key technologies that correspond to its key competences, and leaving to the end users the role of assembling the technologies into a user-specific design. Furthermore, as soon as one of the two design options will appear superior to the other, all CERN resources will be dedicated to it. The project is organised in 5 workpackages (High-frequency hadron linacs, Superconducting magnets, Gantries, Synchrotron design and Machine learning for medical accelerators).