Original: 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.
Simplified: 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.