The DiaCORIS project aims at the construction of a diachronic corpus comprising written Italian texts produced between 1861 and 1945, extending the structure and the research possibilities of the synchronic 100-million word corpus CORIS/CODIS. A preliminary in depth study has been performed in order to design a representative and well balanced sample of the Italian language over a time period that contains all the main events of contemporary Italian history from the National Unification to the end of the Second World War. The paper describes in detail such design processes as the definition of the main subcorpora and their proportions, the type of documents inserted in each part of the corpus, the document annotation schema and the technological infrastructure designed to manage the corpus access as well as the web interface to corpus data.