The Helios Prototype was the fourth and final aircraft developed as part of an evolutionary series of solar- and fuel-cell-system-powered unmanned aerial vehicles.

The larger wing on the Helios Prototype accommodated more solar arrays to provide adequate power for the sun-powered development flights that followed.

The electrically powered Helios was constructed mostly of composite materials such as carbon fiber, graphite epoxy, Kevlar, Styrofoam, and a thin, transparent plastic skin.

The spar, which was thicker on the top and bottom to absorb the constant bending motions that occur during flight, was also wrapped with Nomex and Kevlar for additional strength.

[1] The Helios Prototype shared the same 8-foot (2.4 m) wing chord (distance from leading to trailing edge) as its Pathfinder and Centurion predecessors.

An underwing pod was attached at each panel joint to carry the landing gear, the battery power system, flight control computers, and data instrumentation.

During normal cruise the outer wing panels of Helios were arched upward and give the aircraft the shape of a shallow crescent when viewed from the front or rear.

On August 13, 2001,[1] the Helios Prototype piloted remotely by Greg Kendall reached an altitude of 96,863 feet (29,524 m), a world record for sustained horizontal flight by a winged aircraft.

[1] On June 26, 2003, the Helios Prototype broke up and fell into the Pacific Ocean about ten miles (16 km) west of the Hawaiian Island Kauai during a remotely piloted systems checkout flight in preparation for an endurance test scheduled for the following month.

After a delayed take off, due to the failure of the winds to shift as predicted, Helios spent more time than expected flying through a zone of low-level turbulence on the lee side of Kauai, because it was climbing more slowly than normal, since it had to contend with cloud shadows and the resultant reduction in solar power.

As a result of the persistent high dihedral, the aircraft became unstable in a very divergent pitch mode in which the airspeed excursions from the nominal flight speed about doubled every cycle of the oscillation.