The script for Star Trek: Deep Space Nine‘s third season episode “The Search” called for a new class of warship which it described as “a little clunky.” The vessel “was built primarily for battle,” it said, “not exploration or science.”

The reason for introducing the ship was that executive producer Ira Steven Behr and the show’s writers felt that mere runabouts couldn’t be expected to defend the space station from the Dominion who had shown themselves capable of destroying a Galaxy class starship in the season two finale “The Jem’Hadar.”

“We had all these plans for this Dominion,” said Behr […] “and what were we going to go after them with? Shuttlecrafts?” He added, “You know, it just seemed ridiculous. So we needed a ship.”

Zimmerman’s request for something unique was certainly met: theDefiant was the first Starfleet vessel without external warp nacelles.

Martin didn’t quite recognize the impact it would have at first. “”When you’re in the art department and you’re doing the job from episode to episode, you don’t really think, ‘Boy, this is really going to revolutionize Federation design.’ You’re getting a design out of the way. It’s only after the fact that you think, ‘Wow, that was a different idea.’ I’m glad we took the chance to take a little bit of a departure.”

The name Valiant was dropped in favor of Defiant out of fear that it would conflict with Star Trek: Voyager and its titular starship.

[Source: Designing the Defiant]