The MIDAS Series 2 (Military Defense Alarm System) satellites, MIDAS 3, 4, and 5, were the second development models for the MIDAS early warning system.

MIDAS Series 2 carried a new infrared payload built by Baird-Atomic, one that featured 175 detectors capable of sensing ICBM targets at a maximum slant range of 4200 nm, The payload was designed to scan at a rate of 6 rpm, a rate of rotation three times faster than the Series I payloads.

The Agena-B upper stage of the Atlas-LV3 Agena-B launch vehicle was used as the spacecraft bus and provided power and attitude control to the MIDAS payload. The Agena-B was nearly twice the length of its Agena-A predecessor. The increased tankage and a new dual-burn rocket engine would permit reaching a planned circular polar orbit at an altitude of 3400 km, the orbit then considered most appropriate for an operational constellation of MIDAS satellites. Two deployable solar arrays were mounted on the aft equipment rack of the Agena-B to provide power.

On 12 July 1961, the Atlas booster carrying MIDAS 3 lifted off from Vandenberg AFB and successfully reached a 3400 km circular polar orbit. An hour later they despaired. One of the two solar arrays had failed to deploy properly. Only limited payload data was obtained before a power failure occurred in the Agena. The mission was over after five orbits.