In 1956, Vertol began studies for a new medium-lift helicopter, with the general configuration of the company's HUP-1 Retriever / H-25 Mule helicopter and based on the rotor system of the company's H-21 Flying Banana , but fitted with new lightweight turboshaft engines then in development. The new machine was given the company designation of Vertol Model 107 (V-107) , and company management decided to go ahead with development using their own funds. Construction of the V-107 prototype began in May 1957. The prototype was powered by two Lycoming T53 turboshaft engines, each with 860 horsepower and lent by the US Army. Initial flight of the prototype was on 22 April 1958. The machine was then put through an intensive program of flight demonstrations in the US and overseas. In June 1958, the Army awarded a contract to Vertol for ten production aircraft designated YHC-1A . The Army also issued a request to industry for a larger helicopter, and so Vertol began design work on what amounted to a scaled up version of the V-107, designated the V-114 . The V-114 won the Army competition in March 1959, and Vertol was awarded a contract for a mockup and five prototypes, to be designated the YHC-1B . Vertol was now busy trying to develop two helicopters at the same time, which severely strained the company and led to the Boeing buyout. The number of YHC-1A prototypes was reduced to three (currently under construction) to free up resources for YHC-1B development. The first flight of the initial YHC-1B prototype was on 21 September 1961. The Army had lost interest in the smaller YHC-1A, but Boeing Vertol still felt there was a market for it and continued development. The US Marine Corps was impressed by the machine and awarded the company a production contract in February 1961. The Marines originally wanted to give the V-107 the military designation of HRB-1 , but in September 1962, the Pentagon introduce a new tri-service uniform designation scheme. The HRB-1 became the CH-46A Sea Knight , while the HC-1B became the CH-47A Chinook. The five V-114 prototypes became YCH-47As