Described are a gas turbine engine fan blade platform, related rotor assembly and gas turbine engine, as well as a method of assembling the same. The platform has a forward portion proximal to an axis of rotation, an aft portion, and a transition portion between the forward and aft portions. The forward portion has a forward interface surface facing axially forward, the aft portion has an aft interface surface facing radially outward, and the transition portion has at least one mounting feature. For the method of assembly, an aft support is installed on a fan disk and booster spool assembly. A plurality of fan blades are installed into a fan disk, followed by installing a fan platform between adjacent blades and securing the mounting features to the disk, thereby filling the annulus of the fan disk. Finally, a forward support is installed on the fan disk.