The Kellett O-60 was built and tested as an observation aircraft for the US Army in the early 1940s while the helicopter was still in development. The aircraft had a free-turning 43' 2" rotor which provided lift in place of a conventional wing. A 285 horsepower radial engine turning a two-blade propeller gave the O-60 its forward thrust.

The two-place YO-60 was designed by Richard O. Prewitt and could do jump take-offs. The rotor was spun up to about 280 rpm (rotations per minute) at a no-lift angle using the power of the radial engine. When ready for take-off, the pilot would release a clutch mechanism which changed the blade angle to 8 degrees. This caused the aircraft to 'jump' about 10 feet into the air. The engine and propeller then pulled the YO-60 into forward flight as the rotor angle was decreased to a normal flight pitch of 3 degrees. The engine was only used to spin the rotor up to flight speeds on the ground, while in flight the rotor was free spinning.

Advances in helicopter design in the late 1930s and early 1940s soon made mass production possible and the YO-60 never got past the service test phase. Seven aircraft were built as XO-60 (S/N 42-13604 to 42-13610) with 225 hp. radial engines and later re-engined with 285 hp. radials and redesignated YO-60.

Specifications:
Fuselage length: 20 ft. 9 in.
Fuselage width: 9 ft. 6 in. (at landing gear); 10 ft. at horizontal stabilizer
Height: 10 ft. 3 in.
Rotor diameter: 43 ft. 2 in.
Weight: 2,640 lbs. (gross)
Engine: Jacobs R-915-3 of 285 hp.
Crew: 2

Performance
Top speed: 122 mph