(covers information from several alternate timelines)
The Defiant-class starship, officially an escort vessel, began development around 2367 as a small, highly-powered, heavily-armed warship intended to defend the United Federation of Planets against the Borg. The USS Defiant (NX-74205) was the first of what was to be a new Federation battle fleet. Starfleet abandoned the project when the Borg threat became less urgent; also, design flaws turned up, nearly destroying the Defiant during its initial shakedown cruise.
The Defiant prototype was pulled out of storage in 2371 when the Dominion threat emerged. After several upgrades by Deep Space 9 Operations Chief Miles O'Brien, the ship was deemed worthy of use and more ships of the class were constructed. (DS9: "The Search, Part I", "Call to Arms" , VOY: "Message in a Bottle")
The Defiant-class starship was a heavily-armed, limited-role Starfleet vessel, developed in response to the Borg threat as originally encountered in 2365, and spurred on by the shocking results of the Battle of Wolf 359 in 2367. With the knowledge that existing weaponry and defenses would be inadequate to meet this threat, it was recognized that a new type of starship was needed to meet the tactical requirements of facing this superior enemy. Shortly after the massacre at Wolf 359 in 2367, Lieutenant Commander Benjamin Sisko joined the development team at the Utopia Planitia Fleet Yards. (DS9: "The Search, Part I")
The USS Defiant was the class prototype. It was Starfleet's first dedicated combat vessel, and therefore did not house provision for families or diplomatic missions, nor science labs or recreational facilities.
The Defiant-class was a heavily-armored, stripped-down starship and incorporated the latest in Starfleet weaponry and defensive technology and was equipped with a class-7 warp drive. The Defiant was considerably smaller than most other Federation starships. (DS9: "The Search, Part I")
Starfleet eventually decided to put the Defiant-class into full production. By the end of 2373, there were a significant number of Defiant-class ships in operation, and over the next two years they played a vital role in the Dominion War. (DS9 Season 7; VOY: "Message in a Bottle", "Endgame")
The USS Defiant was destroyed by a Breen warship in 2375, during the Second Battle of Chin'toka. Another Defiant-class ship, the USS Sao Paulo, was then assigned to Deep Space 9. Given the USS Defiant's exceptional record under Captain Benjamin Sisko's command, the Chief of Starfleet Operations gave a special dispensation to change the ship's name to Defiant. On this ship, the shield generators were completely reconfigured so that they would not be affected by the Breen's energy-dampening weapon. This new Defiant played a distinguished role in the final battle with the Dominion at Cardassia Prime, and remained stationed at Deep Space 9. (DS9: "The Changing Face of Evil", "The Dogs of War", "What You Leave Behind")
Knowledge of this class was retained well into the 31st century, as demonstrated when Captain Jonathan Archer and Lieutenant Malcolm Reed viewed schematics on the class in the 22nd century while accessing the database belonging to temporal agent Daniels. (ENT: "Shockwave")
The warp core spanned three decks in the aft section of the ship. The class-7 warp drive was extremely powerful for a ship of this size, and as such, Defiant-class vessels put out a warp signature equivalent to much larger starships. (DS9: "The Search, Part I", "One Little Ship") When energy was diverted from the weapons' ready reserves, a Defiant-class ship could achieve the speed of warp 9.5. (DS9: "The Sound of Her Voice")
A total of four phaser cannon assemblies were located in forward-facing locations on board Defiant-class vessels. The cannons were located above and below the nacelle root attachments on the main body on both port and starboard sides of the ship. (DS9: "The Search, Part I", et al.)
Phaser beam emitters
The standard phaser beam emitters aboard Defiant-class vessels served almost as a secondary supplement to the primary phaser cannon array. In all, the Defiant-class was equipped with at least three emitters: one forward and aft, as well as one behind the bridge on the dorsal surface. (DS9: "Paradise Lost", "Shattered Mirror", "The Emperor's New Cloak"; VOY: "Message in a Bottle")
The Defiant-class vessels' deflector arrays could also be refitted into a makeshift phaser emitter, providing one-shot capability in extreme emergency. The phaser emission was generated by the nose/deflector unit. (DS9: "Starship Down")
Defiant-class vessels had a total of six torpedo launchers, four forward and two aft. The four forward launchers were located on the hull just ahead of each of the warp nacelles in dorsal and ventral pairs. (DS9: "Defiant", "For the Uniform") The two aft torpedo launchers were tucked in near the back end of the warp nacelles. (DS9: "The Die is Cast", "Paradise Lost") These torpedo launchers allowed the ship to fire both quantum torpedoes and photon torpedoes. (DS9: "Defiant", "The Adversary", etc.)
The Defiant-class was equipped with deflector shields.
The Romulans loaned and allowed the limited supervised use of at least one cloaking device. Operation of the cloaking device was to be limited to the Gamma Quadrant and in return, the Federation would provide the Romulans any and all intelligence collected in the Gamma Quadrant and about the Dominion. (DS9: "The Search, Part I", "Visionary") The USS Defiant was the only Federation ship to be seen regularly operating a cloaking device.
Due to the militaristic nature of the Defiant-class starship, the interior design was the most spartan in Starfleet. The Defiant class had five decks.
- Deck 1: main bridge, captain's ready room, transporter room 1, phaser cannons (2), upper main engineering, plasma exhaust vents, upper sensor array, officer and crew quarters, deuterium storage.
- Deck 2: lower main engineering, main impulse engines, computer core, targeting sensors, mess hall, warp coils, sickbay, medical and science laboratory, transporter room 2, officer and crew quarters, warhead control room.
- Deck 3: main impulse engines, warhead impulse engines, shuttlebays 1-3, antimatter storage, cargo bays 1-4, airlocks (2), aft torpedo magazine, warp coils, shuttle and cargo management unit maintenance.
- Deck 4: landing struts, navigational deflector, phaser cannons (2), forward and aft torpedo magazines, aft torpedo launchers (2), forward torpedo launcher (1), lower sensor array, shuttlebay 1-2 exterior doors and elevator system, main tractor beam emitter.
The main bridge acted as the nerve center of the vessel, and the entire module was sunken into a much larger deck 1 than on most Federation starships to provide added protection during combat situations. Access to the bridge was provided by two doorways at the back, located on both port and starboard sides. Just forward of the starboard entryway was the location of the ship's dedication plaque, as well as an auxiliary computer access panel.
The port side of the bridge housed the engineering and tactical I stations, while the starboard side featured the science and tactical II consoles. Because of its nearly exclusive role as a combat vessel, stations aboard Defiant-class vessels were designed with speed in mind.
The center of the bridge featured the lone captain's chair, which was on a raised platform and had a clear view of all bridge stations, as well as of the main viewscreen. On both sides of the command chair were separate control panels, allowing the occupant access to virtually every system aboard the ship.
Between the command chair and the viewscreen was an integrated flight control and operations panel, capable of performing the joint duties of those stations' larger counterparts on other Federation starships. Like all of the bridge stations on Defiant, the conn had been designed for maximum speed and efficiency of commands input by the operator, allowing the craft to be handled almost like a fighter when under the hands of a skilled pilot.
The engineering station allowed for a direct link to the impulse and warp engines, as well as monitoring of the other various systems vital to the operation of the ship. This single station was capable of mirroring all the readouts and consoles located in main engineering, allowing the chief engineer or another engineering officer to issue commands from the bridge.
Mirroring the engineering station on the port side of the bridge was the science station. Normally occupied by the ship's chief science officer, the panels and readouts on this station allowed direct access to the ship's sensor systems. Capable of taking high resolution scans of both natural and artificial phenomena, this station played a vital role during reconnaissance missions.
Flanking both sides of the viewscreen were two tactical stations, which had primary access to the ship's powerful phaser cannons, torpedo launchers and various other special ordinance packages. Working closely with conn, the officers stationed at these consoles were responsible for firing the various weapons aboard Defiant-class vessels during combat operations. The purpose of the Tactical II station was to lighten the load on its sister station, particularly in battle.
Some time after the commission of the Defiant-class in 2371, various aspects of the bridge were redesigned. The captain's chair area was redone to remove the fencing and add new consoles, the dedication plaque was moved to the starboard door area, and additional consoles were added to the side stations and the rear wall. (DS9: "Equilibrium")
Some time in 2373, Starfleet vessels were fitted with new holocommunicator technology. A holographic projector on the bridge could create a holographic simulation of the communique. (DS9: "For the Uniform")
The main sickbay was located on Deck 2 between the mess hall and science labs. Containing four biobeds, this room served as the primary crew support facility during emergency situations. The room was equipped with limited surgical facilities and was primarily intended to stabilize patients until they could be delivered to a nearby friendly starbase medical facility.
In its limited role as a reconnaissance starship, the Defiant-class was equipped with two dedicated science and medical labs for field testing and investigations. These rooms mirrored their counterparts on larger Federation starships by making use of scaled down devices that could commonly be found on those ships. (DS9: "Broken Link")
Main engineering on the Defiant-class was situated on Deck 1 with the lower section on deck 2. The Master Situation Monitor was located in the starboard wall bulkhead. The warp core was placed on an elevated platform in the center of the room opposite the control consoles. Ships of the Defiant-class were equipped with a class-7 warp drive.
Each cabin was equipped with one replicator port and one standard computer terminal. Overall, crew quarters aboard Defiant-class vessels were the most spartan when compared to other ships in the fleet.
With emphasis towards the tactical systems being the foremost priority in the vessel designers' minds, crewmembers had to share their quarters with at least one other crewmate during normal times of operation. Only the ship's commanding officer was given his own room on Deck 1, which doubled as an informal ready room.
Ships of the Defiant-class lacked any sort of recreational facilities, and the only place for informal gathering were the two mess hall areas located on the port side of Deck 2. Like all the equipment and materials aboard the ship, the mess hall was spartan and compact. At the head of the room were three open slots that served as dispensers for the replicators. A counter extended from the underside and was used for the placement of trays, mugs and eating utensils. Seating inside the mess hall was provided by four metallic tables arranged in a semicircle at the wider end of the room. These tables were approximately one meter square, and each had four stools connected to its legs. In addition, the mess hall doubled as a makeshift meeting area that could be used to conduct crew briefings and mission profiles. A tall screen panel located on one of the walls could be used as a visual aid to display tactical graphics. (DS9: "Valiant")
- ENT: "Shockwave" (computer display)
- "The Search, Part I"
- "The Search, Part II"
- "Second Skin"
- "Past Tense, Part I"
- "Past Tense, Part II"
- "Improbable Cause"
- "The Die is Cast"
- "The Adversary"
- "The Way of the Warrior"
- "The Visitor"
- "Starship Down"
- "Our Man Bashir"
- "Paradise Lost"
- "Sons of Mogh"
- "Bar Association"
- "Rules of Engagement"
- "Shattered Mirror"
- "For the Cause"
- "To the Death"
- "Broken Link"
- "The Ship"
- "Nor the Battle to the Strong"
- "Trials and Tribble-ations"
- "The Ascent"
- "The Darkness and the Light"
- "For the Uniform"
- "In Purgatory's Shadow"
- "By Inferno's Light"
- "Soldiers of the Empire"
- "Children of Time"
- "Blaze of Glory"
- "In the Cards"
- "Call to Arms"
- "A Time to Stand"
- "Sons and Daughters"
- "Behind the Lines"
- "Favor the Bold"
- "Sacrifice of Angels"
- "You Are Cordially Invited"
- "The Magnificent Ferengi"
- "Far Beyond the Stars"
- "One Little Ship"
- "Honor Among Thieves"
- "Time's Orphan"
- "The Sound of Her Voice"
- "Tears of the Prophets"
- "Image in the Sand"
- "Once More Unto the Breach"
- "The Siege of AR-558"
- "It's Only a Paper Moon"
- "Prodigal Daughter"
- "Field of Fire"
- "Badda-Bing, Badda-Bang"
- "The Changing Face of Evil"
- "The Dogs of War"
- "What You Leave Behind"
- TNG films:
In the final-draft script for "The Search", the physical appearance of the Defiant-class is initially described thus; "The ship should look a little clunky compared to other Federation vessels we've seen -- this ship was built primarily for battle, not exploration or science." 
According to Martin, the initial instruction for the designers was that it was a "beefy runabout." His initial designs based upon this guideline, however, were not accepted by the producers. Martin next turned to more traditional starship designs, but Zimmerman told him that a completely new look was needed, like nothing ever seen on Star Trek before.
Ultimately, Martin went to a design he had done for a Maquis fighter for the episode "The Maquis, Part II", and he brought it to the producers, who approved it as the basis for the Defiant-class. After tweaking the design a little, Martin gave the schematic to model maker Tony Meininger who, along with his Brazil Fabrications, was given the nod to bring the design to life.
The main problem with the design at that stage was that it didn't look fast enough, so Meininger, a car enthusiast, got some posters of Ferraris, and used them to help him design the 'streamlined' look of the ship, making it very compact and sleek.
Indeed, Herman Zimmerman has stated that the compact design of the Defiant-class influenced the design of the Enterprise NX-01 for Star Trek: Enterprise. He also pointed out, "It was very clear it could not look like NCC-1701-A, B, C, D. It was to be an entirely new ship." (The USS Defiant, DS9 Season 3 DVD special features; Star Trek: The Magazine Volume 1, Issue 4, Star Trek: The Magazine Volume 1, Issue 13; Star Trek: Communicator issue 105)
For DS9, a digital model of the Defiant was built by Digital Muse, which was then borrowed by the visual effects artists of Star Trek: Voyager, as and when required. (Cinefantastique, Vol. 30, No. 9/10, p. 80) A CGI Defiant was also used in DS9's title sequence.
The studio model of the Defiant, measuring 37" × 27", was sold at the 40 Years of Star Trek: The Collection auction on 6 October 2006 for US$85,000 (US$102,000 with buyer's premium).  Additionally, several white foam core camera test models were sold on It's A Wrap! sale and auction, selling at prices ranging from US$27.00 to US$384.00.  Several pieces of Martin's revised sketches of the Defiant, dated 24 November 1994, were also sold on It's A Wrap!, for US$502.00. 
According to Hutzel, during the early development stages of the ship, the Defiant was to be "a new hot battleship that would really set a new standard for an action ship in the series. First, there was lots of talk like 'How big is the Defiant?' Well, we'll make it really small and really powerful, that it would be so powerful and scary, nobody would mess with it." (USS Defiant, DS9 Season 3 DVD special features)
According to Doug Drexler, "The size of the Defiant was a hot potato from day one. Rick Berman wanted a small pocket battleship (a couple times longer than a runabout, with the girth of a regular starship), Gary wanted at least 500' [152.4 m]." 
The scale of the CGI mesh, according to a post in the newsgroup nntp://alt.tv.star-trek.ds9, by DS9 Visual Effects Supervisor David Stipes, was 560' (170.7 m), the length used when he blocked shots in DS9. This was also the size established in the DS9 Technical Manual.
The original MSD that appeared in "The Search, Part I" actually depicted an early concept design and did not represent the final product. It was later replaced with an accurate MSD in such episodes as "Shattered Mirror", although it was used on displays until the end of Deep Space Nine. Drexler, who was responsible for creating the ship's MSD, rationalized "a 420′ [128.0 m] Defiant." 
For further analysis on the Defiant's scaling, visit the "Defiant Problems" link at Ex Astris Scientia below.
Initial drawings described the class as the Valiant-class. Indeed, Drexler even constructed an initial dedication plaque for the Defiant that referred to it as the "Valiant-class," before a new plaque was recreated that simply stated "the first ship of her class." The plaque was later given to Rob Bonchune. 
The interior sets of the Defiant were supervised by Herman Zimmerman.
According to regular background performer Brian Demonbreun there were special carpets transferred onto the "normally" seen carpet of the Defiant in scenes including explosions and sparks. All actors, mostly the background performers, who were near those sparks were made wet, with a light moisture film on their hair and uniform.
The following details and specifications comes exclusively from the Star Trek: Deep Space Nine Technical Manual:
- Production Base: ASDB Integration Facility, Antares Fleet Yards, Antares IV
- Type: Escort (officially); Warship (unofficially) (DS9: "The Search, Part I")
- Accommodation: 40 officers and crew; 150 personal evacuation limit
- Power Plant: One 1,500 plus Cochrane warp core feeding two nacelles; two impulse modules
- Dimensions: Length, 170.68 meters; beam, 134.11 meters; height: 30.1 meters
- Mass: 355,000 metric tonnes
- Performance: Warp 9.982 for 12 hours
- Armament: Four pulse phaser cannons; two torpedo launchers
The notched forward hull containing the vessel's navigational deflector and main sensors was a detachable single-use last-resort missile. This missile was equipped with a small impulse engine and a magazine with six torpedo warheads. These torpedo warheads were part of the ship's auto-destruct system and under more normal circumstances could be fired from the forward launcher. Any torpedo warheads left in the magazine would become the missile's warhead upon the missile's separation from the rest of the ship. If no torpedo warheads remained, then the missile would have to rely on kinetic energy alone. The ship would be unable to travel at high warp speeds without the missile since the latter contained the ship's navigational deflector.
During the mission "ambush" in the game "Star Trek: Legacy" the USS Defiant was sent to the "Etarian Nebula" for a test flight of its systems, where the crew unfortunately found that the Romulans had made an invasion of the nebula waiting for a fugitive, the Enterprise-D was sent to rescue the Defiant with 3 other starships and to neutralise any Romulan threat remaining.
- The William Shatner series of novels featured another Defiant-class vessel, the USS Monitor. The Monitor boasted several modifications from a standard Defiant-class starship, such a cloaking device and black sensor reflective hull plating. It's warp engines were tuned so the nacelles would emit ultraviolet light instead of the standard red and blue, and the ship had no running lights or hull markings. In Shatner's novel The Return, the Monitor is temporarily renamed Enterprise during a mission to the Borg homeworld using a stolen transwarp drive, commanded by Captains Kirk, Picard, and Spock.
- The Decipher role-playing supplement Starships lists many other conjectural vessels of this class including the USS Gallant (NCC-74206), USS Incessant (NCC-75636), USS Redoubtable (NCC-75634), and USS Stalwart (NCC-75635).
- Star Trek Online has Defiant class vessels available for characters who reach Captain rank or higher as a Tier 4 "Tactical" Escort or Tier 5 "Tactical Escort Retrofit" (the difference being that the Tier 5 variants come equipped with cloaking devices). Other Tier 4 escorts are meant as late 24th and early 25th descendants of the Defiant class, sharing its basic profile -- these include the Vigilant class and Gallant class Tactical Escorts. It also has a refit version called the Sao Paulo class "Tactical Escort Refit" with Quad Phaser Cannons. The game features two NPC Defiant class starships, the USS Defiant and the USS Belfast.
- In both Star Trek: Armada and Star Trek: Armada II the ship, in particular the Avenger, carries antimatter mines.
- A Defiant-class starship appeared as an escort in the video game Star Trek: The Next Generation - Birth of the Federation.
- The Defiant-class is playable in multiplayer and skirmish modes in the PC and Xbox 360 video game Star Trek: Legacy.
- Star Trek: Armada II features the following possible names for Defiant-class starships:
- Defiant class starship at Wikipedia
- Template:Exastris - a comprehensive look at the scaling issues of the Defiant-class
- wbm - an article by Jim Martin taken from the August 1999, Issue 4 and May 2000, Issue 13 of the Star Trek: The Magazine
- USS Defiant at Cygnus-X1.net, the Star Trek LCARS Blueprints Database