Development began in the 1950s by the Reaction Motors Division of Thiokol Chemical Company to power the North American X-15 hypersonic research aircraft.

It could deliver up to 57,000 lbf (250 kN) of thrust with a specific impulse of 279 s (2.74 km/s) or 239 s (2.34 km/s) at sea level.

Operating times nearly twice that were recorded in tests, but declared largely unsafe.

Like many other liquid-fuel rocket engines, the XLR99s used regenerative cooling, in that the thrust chamber and nozzle had tubing surrounding it, through which the propellant and oxidizer passed before being burned.

The LR-99 was used exclusively to power the X-15 research aircraft after initial trials that used a pair of Reaction Motors XLR11s.