JF is correct.
Check out the Myasishchev VM-T Atlant as well:
VM-T Atlant Description
MAP tasked the OKB-473 design office (Antonov) to design this plane. By then the work on the AN-124 'Ruslan' was well advanced, so Antonov took the decision to build this new plane (
AN-225) based on the design of the AN-124. Unfortunately much time was wasted in discussions with Antonov and the prototypes of the Energia LV and Buran were nearing completion in the mean time.
Seeing that the Antonov design office had still troubles with developing its plane the Myasishchev design office suggested to modify the bomber 3M to carry those loads and renamed it 3M-T (T for Transport). Other design offices suggested solutions but they were soon dropped, using an IL-76 Candid, or two helicopters Mil-Mi 26 (the carriage of the tanks proved to be too hazardous). The transformation project consisted of using 3MD model (bomber with refueling probe) by changing the rear part of the fuselage. Indeed, the transport of a load like the Shuttle or Energia's tanks makes a lot of turbulences at the rear of the plane, they should replace the single vertical stabilizer by a twin tail. Many tests in wind tunnels were performed and proved this design to be reliable.