The cathedral's work began in 1296 but was discontinued in 1348 because of the infamous Black Death that had hit Italy that year.
And though the cathedral was structurally completed by 1436, if my knowledge serves me right, it was an architectural challenge to execute a Duomo as gigantic without supporting it on any pillars, for that era. And hence, was announced as part of a competition on 19 August 1418, where Brunelleschi won and received the commission.