In the years up to 1623, Papirio Bartoli, Cardinal Federico Borromeo’s Milanese agent in Rome, drew up a project for an intervention on St. Peter’s Basilica in the Vatican. The Lombard’s main proposal was the construction of an enormous boat-shaped choir at the crossing of the church, but the dilettante also suggested the construction of two new aisles, a new façade, and a porticoed piazza in front of it. This essay analyzes the iconographic components, the liturgical sources, and the models of Bartoli’s project. It furthermore studies the culture of Lombard and Hispanic origin on which his ideas drew in order to understand the development of the navicella, viewing it not so much as a real proposal but as one more piece in the artistic theology of the seventeenth century.