Originally developed and manufactured for the United States Army by American defense contractor West Tek, the T-45d power armor was the first version of power armor to be successfully deployed in battle.

In 2076, the more advanced T-51b power armor variant was introduced into combat duty. After the Great War, this model of power armor is still being used by factions such as the Brotherhood of Steel. While main forces of the Brotherhood in New California and the Mojave Brotherhood of Steel tend to use the newer T-51b, more remote chapters, such as the East Coast Brotherhood of Steel and the Brotherhood Outcasts, have to rely on these outdated suits.

Design
The T-45d lacks the mobility of later power armor models (such as the T-51b power armor) because of its less advanced servo design; as a result it limits the wearer's Agility.

While the T-45d armor is similar in its overall shape to the newer T-51b power armor, it is significantly less advanced than its later counterpart, and uses riveted steel plates instead of plastic composite materials in its construction. Various electronic and mechanical subsystems of the armor are exposed on the outside of its casing rather than sealed within as in the T-51b, making it far more susceptible to damage. In addition, a specialized body suit is required to be worn underneath the T-45d's armored shell in order to facilitate linking the armor's servos to the wearer's own motions and to make the armor more bearable because of its heat dissipation problems.