Since the dawn of vertical-to-horizontal flight offered by helicopters (the first operational models were had during World War 2), aeronautical engineers have strived for ways to incorporate even better straight-line performance. This led to a myriad of experiments seen throughout the Cold War period as companies attempted to find a proper solution. For a time, "pusher" propellers seemed the future while other designs relied on complex turbojet arrangements coupled to traditional helicopter blade arrangements. In time, the technology behind "tilting" nacelles was refined, leading to the concept of the "tilt-rotor" aircraft - an air vehicle that was part helicopter, part airplane.
In time, Bell worked on bringing its XV-3 prototype to life which followed the Transcendental "Model 1-G" and "Model 2" forms. The Model 1-G was the first tilt-rotor in aviation history to fly and was powered by reciprocating engines buried in the fuselage driving power, via shafts, to wingtip rotors that could tilt. The subsequent XV-3, a more evolved form of the Model 1-G and Model 2, reached the skies in 1955 using similar concepts and this aircraft went on to hold the distinction of becoming the first tilt-rotor aircraft to convert from vertical-to-horizontal flight, paving the way for the refined XV-15 development that followed.
The XV-15 was a radical redesign of the same tilt-rotor approach but relocated its engines to tilting nacelles placed at the wingtips. Between the two engines was a shared driveshaft to be relied upon should one engine fail - the other could keep up the workload. The XV-15 project was formally launched in 1971.
The basic arrangement of the XV-15, which is mimicked today by the in-service V-22 of the American military, was set: the fuselage held a helicopter-like form with side-by-side seating at the cockpit for two crewmen while the aft-section was tapered. Over the roof of the vehicle was seated the shared wing component to which the tilting engine nacelles were seated at the tips, these driving large, thick rotor blades about an oversized spinner. The fuselage was braced on the ground by a retractable tricycle arrangement and the tail incorporated a twin-fin rudder configuration set on a shared horizontal plane.
The XV-15 was flown for the first time on May 3rd, 1977 and, itself, was the progenitor to the Boeing V-22 "Osprey" line detailed elsewhere on this site. The Boeing V-22 became the first tilt-rotor aircraft to formally enter operational service n 2007.
The U.S. government contracted for a pair of flyable prototypes to continue the program and competition was had from Boeing, Grumman, and Sikorsky joining Bell. For NASA, the Bell submission ultimately won out with its Model 301 when going head-to-head against Boeing (which supplied its Model 222). Bell then produced two prototypes, N702NA and N703NA. These went on to be extensively tested at the Ames Research Center in Mountain View, California and then to Dryden (NASA, Edwards AFB) for their time in the air. It made its public debut at Paris Air Show 1981 and proved itself a hit with onlookers.
N702NA later crashed on a test flight (no loss of life) with its remains reconstituted for the simulator role. N703NA survived its flying days to become a display, first, at the National Museum of the United States Air Force in Dayton, Ohio and then, later, a display at the Udvar-Hazy Center aviation museum near Washington, D.C.
With its usefulness over, the XV-15 series was retired in full in 2003.
Bell teamed with Boeing to bring about the V-22 series, a larger version of the XV-15 with greater power and capabilities.