Tension with neighboring Honduras was building up in 1968.  At the time, Salvadoran armor was sparse. Given this lack of armor, particularly lack of armored transports, in 1969 Maestranza de la Fuerza Armada de El Salvador (FAES) built five gun trucks to be used as “fast assault” mobile platforms against barricaded Honduran troops. Maestranza came up with a simple design consisting of an armored body mounted on a REO M34/M35A1 2½-ton truck chassis. The body consisted of spaced armor, with an out-layer of armored plate, followed by cotton-fiber fill, and then an inner armored plate.  The vehicle was open topped, allowing its use as transport and as weapons platform.  The name REO inspired the gun truck name of RAYO (Lightning), which in turn visualized the main role as an “fast assault” vehicle.

By 1972, the Salvadoran Army had produced up to 20 Rayos, and a number of them were slightly modified as mortar carriers, equipped with Yugoslavian 120mm UBM-52 mortars.  However, all the Rayos had been taken out of service by 1975, and started being replaced by the new Mazinger in 1979.