The game follows Sam "Serious" Stone traveling through various historical settings to destroy the horde of his nemesis, Mental, and its teleportation beacons.

As Sam, the player navigates 2D levels and can use the "gun stacker" to create vertical stacks of up to six weapons that are fired simultaneously.

[3][4] The primary objective is leading Sam "Serious" Stone through various historical settings, fight enemies from the horde of his nemesis, Mental, and destroy teleportation beacons it placed.

[6] Sam, as the player character, traverses confined levels, most of which have a primary route leading to the right, while others have branching paths, and some include puzzles.

[7] Enemies approach from multiple directions and attack using varying behaviors: some engage in melee combat, others bear ranged weapons, and some act as suicide bombers.

[13] For the local multiplayer mode, a second player, assuming the role of Dan "Huff" Huffington, can join in at any time and engage in cooperative gameplay.

The publisher sought a "Contra on acid", while Fouts ruled out simply creating a 2D conversion of a previous Serious Sam game.

[16][17] Fouts modeled Double D after Weapon of Choice, re-using and improving on its development tools and XNA-based game engine.

[16][18] Using a prototype image and specifications for the "gun stacker" mechanic, he pitched the idea to Devolver Digital, which replied with "Make this right now!

[20][21] He was joined by three collaborators: Arizona-based Hamdija Ajanovic composed original music, Florida-based AJ Johnson wrote the story and dialog, and Richard Rosenthal provided further programming.

Mommy's Best Games and Devolver Digital agreed on features that would make such a port worth releasing, which Fouts subsequently developed, following the content specifications for the platform.

[39] By March 2012, Fouts was deeply engaged in creating new game art for a "(sorta) secret project" relating to Double D.[40] Voice-over was added for all characters, with John Dick reprising his role as Sam.

He eventually agreed with Michael Dalmon, the actor from the trailers, to use his voice and likeness, adding his character to the game as Dan "Huff" Huffington, along with additional dialog between him and Sam.

[33][41] To combat repetitiveness and adjust the game's pacing, Fouts created more levels that required less shooting, of which some used vehicles and others necessitated strategizing.

[45] In a two-part "Gun Diary" series of videos publicized between January and February 2013, Fouts explained the additions made to XXL.

[39] In June 2013, he started porting the game to the platform, cleaning up hard-coded elements, and after a break continued development in September.

[53] XXL was released on Steam on October 21, 2013, as a free update for existing owners of Double D, which remained playable separately.

[1][3][6][7][9][14] James Stephanie Sterling (Destructoid) called it "clever", saying that it added a "perfect level of freshness" to the Serious Sam formula, which Iain Farquhar (Bit-Tech) opined had been adapted faithfully.

[3][6][14] According to Rich Stanton (Eurogamer), the shooting in the game was "quantity over quality" and did not feel good enough to make up for its extensive usage.

[61] David Sanchez (GameZone) opined that the level design was "nicely varied",[15] although Parkin called it "scattershot and somewhat amateurish".

[3][7] Hancock labeled the unlockable challenges as "interesting" due to their limited arsenal, while Farquhar opined that they added little content beyond the campaign.

The writing received conflicting responses: Sterling called the humor "genuinely amusing",[14] Onyett characterized it as "childish" but found it fitting for the game's tone,[1] Sanchez described the writing as "pretty sharp and filled with genuine laugh-out-loud moments",[15] and Mejia expressed general enjoyment.

[7][61] Farquhar experienced "mushy and unresponsive" controls and input lag in Double D;[8] long loading times and occasional frame rate issues were reported for the Xbox 360 version of XXL by others.