The Duomo, or as the cathedral is actually the called, the Santa Maria del Fiore (St. Mary of the flowers) was constructed to replace the much smaller and worn Santa Reparata, as you can see on this layout.
You can still see remnants of this old church in the Duomo if you descend down the stairs at the front of the nave. The architect and sculptor Arnolfo di Cambio made a design for a new cathedral. What’s more, Cambio also sculpted sculptures for the facade (click here to see them) that we will discuss on the day we visit the Opera del Duomo museum. It speaks for itself that this church was to be as beautiful as possible (più bello che si può). That is according to a document from 1294, the same year the first stones of the Santa Maria del Fiore were placed, which said the following: ‘one must not endeavour in public works (cose del Commune) if the plan does not match a desire fueled by the zeal of its many citizens that are united by a common cause.’
The construction firm tasked with the construction, the Opera del Duomo, prioritised the Palazzo Vecchio instead. Another factor is that Arnolfo di Cambio died pretty quickly. It was not until 1334 that the new master builder, Giotto, got the project going again, but his main focus was on building the campanile, the clocktower next to the Santa Maria del Fiore. In 1357, master builder Francesco Talenti appears who changes Cambio’s original design. Originally, the design included rectangular bays, but Talenti turned them into square bays, which significantly increased the size of the church (see layouts at Wikipedia). He also raised the walls of the aisles. This created a problem with the already constructed part in the aisle, which had two windows in the bay. The new construction part was only to be given one window per bay. In the old part of Arnolfo di Cambio, the windows no longer match the interior and eventually they were made into blind windows. This can still be seen on the exterior of the aisles today.
The spacious room covered by the high arches gives the interior a completely new look than gothic churches north of the Alpes. The aisles and the nave appear to be one large space to the naked eye. Master builder Giovanni di Lapo Ghini finalised the Duomo and in 1367 the cathedral was completed (the walls and vaults), save for the dome and the tholobate. For more info about the Duomo click here for Wikipedia.