XH-40 and YH-40
The first Bell helicopter to use a turbine engine was a modified Model 47 (designated XH-13F), which had its initial flight in October 1954. The U.S. Army began a competition for a new helicopter for general utility and medical/casualty evacuation in 1955. In June 1955, Bell Helicopter was awarded a contract to develop the next generation turbine-powered utility helicopter for the U.S. Army. The resulting Bell Model 204 was designated XH-40 by the U.S. military and first flew on 22 October 1956. Two more prototypes were built in 1957, and six YH-40 pre-production helicopters were delivered in 1958. The YH-40's cabin was lengthened by12 in (30 cm), and had more ground clearance compared to the XH-40.
Bell believed the YH-40 was ideal for troop transport and cargo carrying as well as the medevac role, a view soon adopted by the Army, who found the pre-production aircraft so much better in service than previous piston-powered helicopters they soon ordered more.
Read more about this topic: Bell UH-1 Iroquois Variants