the design bramante proposed was a radical break with history and tradition, one fundamentally different from earlier church designs. he proposed replacing constantine's ancient basilican building with a centralized domed structure. peter's tomb would no longer be at the end of a long apse, but in the centre of the church under an enormous dome resting on four immense vaulted halls equal in length, forming a greek cross. a radical composition of elements, bramante's proposal pivoted around a central point, and in it, labels like atrium, nave and transept no longer applied due to its highly generic design. the idea of a centrally planned domed church had been present in written descriptions and in the background of paintings for decades by the time bramante was designing st peter's. the most canonical reference to the central plan is probably alberti's writing on the topic, in which the architect states that the principal church of a city, its 'temple', ought to be an immense radial domed structure sitting on a plinth in the middle of a plaza. writers and artists, rather than architects, described and depicted this concept long before anyone actually built it. thus, bramante's interest in the materialisation of this architectural idea made st peter's an 'ideal' church.
forster, a. c. (2014) bramante's half plan, essay