The airframe of the Model 120 was very simple, comprising a welded steel-tube open structure, with the three-bladed main-rotor mast and gas-producers attached without covering.
Rotor drive was by compressed air rotor-tip jets, fed by three 200 hp (149 kW) gas power AiResearch GTC 85-135 gas-producers.
[2] The Model 120 was only ever intended to carry loads under-slung or attached directly to cargo hooks on the underside of the top fuselage beam, including specialised pods.
Although aimed at the US Army the Model 120 was also evaluated by the US Navy at the Naval Air Test Center (NATC), NAS Patuxent River, in September 1959.
The Model 120 experienced powerplant problems initially, but demonstrated an excellent load to weight ratio of 1.5:1, but despite the proven performance no orders were forthcoming and cancellation of the project in February 1960 signalled the end of McDonnell's helicopter aspirations.