One important early concept was associated with the company’s “220-2” design. It demonstrated the basic principle that would eventually become central to the F-35B: use the aircraft’s primary engine to mechanically drive a separate lift fan.

This approach represented a significant departure from traditional STOVL arrangements.

An aircraft could theoretically use a dedicated lift engine or multiple engines to provide vertical thrust. But doing so would add weight and occupy valuable internal space. The aircraft would then carry propulsion equipment that might be used primarily during takeoff and landing rather than during normal cruise.

The lift-fan concept offered another possibility.

Instead of carrying a completely separate engine, the main engine could provide mechanical power to a fan located toward the front of the aircraft. During conventional flight, the fan would not be used for vertical lift. During STOVL operations, however, power from the main engine could be transmitted through a driveshaft and gearbox to the fan.

This arrangement allowed a single propulsion system to perform two fundamentally different tasks.

The engine could generate conventional forward thrust during normal flight while also supplying mechanical power for vertical lift when required.

That idea became one of the defining characteristics of the F-35B.