A hybrid-electric powertrain system for aircraft includes a gearbox having a first rotary shaft for output to drive an air mover for aircraft thrust. The system includes a first prime mover connected by a second rotary shaft to the gearbox for power input to the gearbox. Further, the system includes a second prime mover connected by a third rotary shaft to the gearbox. The second prime mover can have a hollow core, and at least one of the first and second rotary shafts passes through the hollow core and the third rotary shaft.