When Pope Julius decided to build the `` grandest church in Christendom '' the design by Donato Bramante was chosen , and Pope Julius laid the foundation stone in 1506 . Pope Julius ' scheme for the grandest building in Christendom was the subject of a competition for which a number of entries remain intact in the Uffizi Gallery , Florence . It was the design of Donato Bramante that was selected , and for which the foundation stone was laid in 1506 .