Design Nostalgia:
Bell evidently allocated design numbers (prefixed with a "D") for their various projects. This was D-18, an early four bladed design. It looks as though it should be contra-rotating but I don't think it is, hence the tail rotor.
This project was designated D-1007 and was intended to be nuclear powered, to fly at 175 kts and was some 300ft long!
Ah, atomic helicopters .. those were the days!