It was a project spearheaded by Commander (later Captain) Sisko, who wanted to build a ship specifically designed for combat against the Dominion.
It was also designed with input from the Breen Consortium, who were initially hostile to Starfleet but later agreed to help with the ship's construction.
It's worth noting that the Defiant's design was controversial, as it was seen by many as being too aggressive and militaristic for Starfleet. However, its effectiveness in combat against the Dominion ultimately vindicated its creation.