The Bell YOH-4 (originally YHO-4) was a single-engine, single rotor, light helicopter developed for the United States Army's Light Observation Helicopter program. While the YOH-4A was unsuccessful in the original LOH competition, Bell redesigned it as the sleek Bell 206A JetRanger for the commercial market, and enjoyed instant and lasting success. In 1967, the Army reopended the LOH competition, and the 206A-based OH-58 Kiowa was selected.
In October 1960, the Army submitted a request for proposals (RFP) for the Light Observation Helicopter (LOH). Bell, along with 12 other manufacturers (including Fairchild-Hiller and Hughes Tool Co. Aircraft Division), entered the competition. In January 1961, Bell proposed their Model 206 design, which was selected out of the design phase of the Navy-run competition by the Army and designated as the YHO-4A.
Bell produced 5 prototypes in 1962 to submit to the Army for the Test and Evaluation phase, the first prototype making its maiden flight on 8 December 1962. That same year, all aircraft began to be designated according to the new Joint Services designation system, so the prototype aircraft were redesignated as YOH-4A. During the flight testing phase, the power problems of the YOH-4A eliminated it from consideration for selection.
After the failed military contract bid, Bell attempted to market the Model 206, but it didn't fare well at all commercially. Bell's market research showed that customers found the body design mostly unpalatable. Bell redesigned the body of the airframe to a more sleek and aesthetic design and reintroduced it as the Bell 206A JetRanger, and Bell found they had a commercial success on their hands.
In 1967, the Army reopened the LOH competition for bids because Hughes Helicopters couldn't meet the contractual production demands. Bell resubmitted for the program using the Model 206A. Fairchild-Hiller did not resubmit their bid with the YOH-5A, which they had somewhat-successfully marketed as the FH-1100. In the end, Bell underbid Hughes to win the contract and the Bell 206A was designated as the OH-58A.