There was a contest and Bramante won it (1506) with this design:
It was a simple (Greek) cross inside a square. The arms protruded slightly and were rounded off in apses.
That big circle in the middle was the dome over St. Peter’s tomb. It would be the biggest in the world.
There were four little domes between the arms of the cross.
Bramante died before construction was far along. Over the next forty years other popes ordered other architects to improve Bramante’s design but none did. Finally, Paul III gave the job to Michelangelo.
Michelangelo Takes Over
He studied all the designs and decided to keep Bramante’s. He knew a good thing when he saw it, even if it was the work of an old enemy of his. The present-day sanctuary or east end of St. Peter’s is essentially Bramante’s with Michelangelo’s alterations.
Michelangelo’s design (1546-1564)
Just what alterations did Michelangelo make?
He decorated. He added endless moldings and wall decorations, windows and niches. He gave the apses a slightly greater bulge and then put in a bridge wall between them and the corners of the square. He hid the simple shape of the building by putting a sort of folding screen around it, with lots of panels, tying them all together with a huge “ribbon” (the cornice).
This is what St. Peter’s now looks like from the back, where Michelangelo’s design is best seen.
None of that was built while Michelangelo was alive, however. Construction was only as far as the dome–or rather, the drum for the dome.
Here is Vasari’s painting of its construction, also seen from the back. That’s Pope Paul III giving orders.
The cupola design (the cupola is the roof over a dome) gave Michelangelo a lot of trouble. An assistant helped him with the wooden model.
Michelangelo’s wooden model, altered by Giacomo della Porta
Is his the same as the present-day cupola?
Almost. Michelangelo’s was hemispherical. A later architect, Giacomo della Porta, stretched it, made it taller.
At Michelangelo’s death only the drum (round base for the dome) was completed. It was still attached to the old fourth-century basilica.
An engraving showing St. Peter’s from the front six years after Michelangelo’s death
Eventually Old St. Peter’s was torn down to make room for the big naves and facade that were added by Carlo Maderna. This is his, the final floor-plan.
was based on the old Roman Pantheon’s:
Bramante had also been thinking of the Pantheon when he came up with the original design for St. Peter’s.