The An-325
The An-325 was a planned, larger and more capable version of the An-225 Mriya, that never made its way into service.
The plane was proposed to use two extra engines, which would have been mounted on the inner engine mounts. It would have been an eight-engine aircraft with six engine nacelles.
It was supposed to be used as a launch platform for both russian and foreign spacecraft. It would have been the largest and most capable plane ever.