In the place where the cathedral once stood were the ancient cathedral of Santa Maria Maggiore, winter cathedral, and the basilica of Santa Tecla, summer cathedral. After the collapse of the bell tower, Archbishop Antonio de’ Saluzzi, supported by the population, promoted the reconstruction of a new and larger cathedral (May 12, 1386), which stood on the site of the oldest religious heart of the city. For the new building both previous churches began to be demolished: Santa Maria Maggiore was demolished first, Santa Tecla later, in 1461-1462 (partially rebuilt in 1489 and finally demolished in 1548)
The new church, judging by the archaeological remains that emerged from the excavations in the sacristy, was originally to provide a brick building according to the techniques of Lombard Gothic.
In January 1387 the foundations of the pylons were thrown, colossal works that had already been designed to design the previous year. During 1387 excavations of the foundations continued and the pylons continued. What was done before 1386 was all or almost undone. During the year the Lord Gian Galeazzo Visconti took control of the works, imposing a more ambitious project.
The material chosen for the new construction then became the marble of Candoglia and the architectural forms those of the late Gothic of Rhine-Bohemian inspiration. Gian Galeazzo’s desire was in fact to give the city a grandiose building in step with the most up-to-date European trends, symbolizing the ambitions of his state, which, in his plans, should become the center of an Italian national monarchy as had happened in France and England, thus becoming one of the great powers of the continent. Gian Galeazzo made the quarries available and granted strong subsidies and tax exemptions: each block destined for the Duomo was branded AUF (Ad usum fabricae), and for this reason exempt from any passing tribute. As evidenced by the rich archive preserved to this day, the first chief engineer was Simone d’Orsenigo, flanked by other Lombard masters, who in 1388 began the perimeter walls. In 1389-1390 French Nicolas de Bonaventure was commissioned to design the finestrones.
French and German architects, such as Jean Mignot, Jacques Coene or Henry of Gmünd, were called to lead the construction site, but they remained in office for a very short time, encountering a discovery of hostility on the part of the Lombard workers, accustomed to a different practice of work. The factory then went on in a climate of tension, with numerous revisions, which nevertheless gave rise to a work of unmistakable originality, both on the Italian and European scene.
Initially the foundations had been prepared for a building with three naves, with square side chapels, whose dividing walls could also be buttresses.
It was then decided to do without the chapels, bringing the number of naves to five and on July 19, 1391 the enlargement of the four central pillars was decided. However, there was growing concern about the stability of the entire structure, due to insufficient inertial masses to be contrasted with the action of thrusts. Thus in September of the same year the Piacentine mathematician Gabriele Stornaloco was questioned to define the cross section and the raised, through a precise geometric and cosmological diagram (stornaloco was also an astronomer and cosmographer).
On May 1, 1392, the progressively decreasing naves were designed for a maximum height of 76 arms.