 The D-255 was planned to be a purpose-built attack aircraft based on the UH-1B airframe and dynamic components with a new, slender airframe and a two-seat, tandem cockpit, featuring a grenade launcher in a ball turret on the nose, a 20 mm belly-mounted gun pod, and stub wings for mounting rockets or SS.10 anti-tank missiles.A proof-of-concept contract was awarded to Bell in December 1962 for the Model 207 Sioux Scout, which was, in essence, a new forward fuselage and Bell 47G-3 dynamic parts fitted to a Bell 47J center and rear fuselage.The Sioux Scout included all the key features of a modern helicopter gunship – a tandem cockpit, stub wings for weapons, and a chin-mounted gun turret.