Bone (lost production material of cancelled film adaptations of Jeff Smith comic series; 1998-2000; 2008-2016)
Bone is a fantasy comic book series created by Jeff Smith and published by Cartoon Books that ran from 1991 to 2004 for 55 issues. It tells the story of the Bone Cousins (Fone, Phoney, and Smiley) who are ran out of Boneville and end up in The Valley. With the help of Thorn and her grandmother, Gran'ma Ben, the cousins try to get back to their home. All the while, they're caught up in trials and tribulations through the valley, and even a war. The series has become a beloved cult classic among readers, with many attempts having been made to adapt the series into a film, however, none have been successful. Only a handful of material for either adaptation of the comic has surfaced.
Nickelodeon Movies/Paramount Pictures version (1998-2000)
In August 1998, Jeff Smith signed a deal with Paramount Pictures and Nickelodeon Movies to produce an animated adaptation of Bone. A first draft of the script was written a year later, with Smith stating that pre-production may start in 2000, with him to lead as director. In the summer of 2000, as reported by Corana Coming Attractions, it was to be traditionally animated by Character Builders (Smith's now defuncted animation studio) and with a tentative release set for 2002. However, while Nickelodeon and Paramount were both very high on producing it in meetings, they wouldn't return any phone calls from Smith afterwards, and had the choice to let the Bone film rights expire later that year, which would foreshadow the eventual fate of the film. The story would've followed the Bone Cousins ending up in the valley, where they would meet Thorn and Grand'ma Ben. Smith pitched the story to be very simple, but with an open ending that could potentially lead to more sequels for the future.
However, Nickelodeon wanted to change the ending and make the film more kid-friendly. Most notably, they wanted the Bones to be played by children, and for Fone Bone to have "Magic Gloves". They also wanted to shift more focus on the character Thorn, and even debated focusing the movie on her instead. There were also talks of pop songs by NSYNC and Britney Spears to be featured in the films soundtrack, which was quickly turned down by Smith. The executive meddling caused Smith to walk out of the film, resulting in production being cancelled in August of 2000. But in a September 2000 interview with the Comics2Film website, Smith said even after the film's initial end the prior month, there was still a chance for it to be made, as he stated:
"I'm definitely not running away from it. In fact we're still talking. I think things are good. I'm not frustrated in any way. We're just taking a breather. I just had to hit the reset button."
By December 2002, in an IGN interview where he is asked 10 questions about himself, Smith said he shelved the idea of an animated Bone at the time, seemingly killing any hope for the original film version to be produced. 
Warner Bros. version (2008-2016)
Nearly eight years after the previous version was cancelled, it was revealed in March of 2008 that a new Bone film was in production. This time, it was to be distributed by Warner Bros. Pictures, with Dan Lin producing, and Smith coming on board as an executive producer. Animal Logic (known for their Oscar-winning film Happy Feet) and Lin Pictures were to produce, as it was later announced in 2009 that the film was instead going to become a CGI trilogy,  with motion capture animation being provided by the former.  The first film in the proposed trilogy would have been based on either the first three or four books with a potential PG-13 rating attached and was to be written by Farscape writer Justin Monjo. A four-minute animation demo reel was shown to Smith in summer 2011, who described it as:
"Fone Bone was falling in the water and going through cliffs and canyons. The dragon moved in from off-camera in the shadows with smoke around him, all in 3-D. It was pretty mind-blowing."
In January 2012, Greek creator Patrick Sean Smith was brought on to write from working on a previous draft by Monjo, and P.J. Hogan signed on as the director with a projected release date in either 2013 or 2014.  However, no updates on the for the film came for nearly five years, and in November 2016, Mark Osborne (Kung Fu Panda) and Adam Kline, were revealed to direct and write the film trilogy, effectively replacing Smith and Hogan. There has been no further news on the project since, and with the announcement of a Bone animated series coming to Netflix in October 2019, the project is all but certain to be shelved. However on April 20th, 2022, Netflix cancelled their attempt altogether, leaving the series' future to become animated in question. 
Prior to that cancellation in December 2021, it was revealed that at one point in development, Osborne was joined by Kubo and the Two Strings writer Shannon Tindle and El Tigre and The Book of Life creator Jorge R. Gutierrez to write the trilogy simultaneously, with the plan to direct one of the three films each.  It was implied the adaptation was briefly considered to be moved to 20th Century Fox, but any plans fell through once Disney's acquisition of the company was fulfilled in 2019. The tweet remains the only public mention of an alleged Fox version. 
Up until the early 2020s, no material from either version of the film had surfaced. But it was discovered on August 16th, 2020 by Reddit user Clean-Cupcakes on r/Bone that Paramount concept art was featured on a defunct animation website called Animated Movies, on their page about the Bone film.  Though its initial legitimacy was questioned, fellow Reddit user Lokigere revealed further details, and was able to prove that the art was legitimate. The concept art was featured in a collection of novels for the series titled Bone Vernal Equinox Slipcase Edition, and also contained another piece of concept art for the film. A day later, the user uploaded both images, albeit in low quality. 
On February 6th, 2021, Lost Media user Fun.Tendo was able to find a high-quality version of the second image presented in the Bone Vernal Equinox Slipcase Edition, via using the Wayback Machine to search for a September 2000 Smith interview with Comics2Film mentioned in the Animated Movies page about the film. In the article, the previously unknown artist behind both images was revealed to be Neil Ross (a U.K. concept art designer that has worked on plenty of films, including Braveheart and Spider-Man: Into the Spider-Verse) and apparently both of the known concept art were shared to Comics2Film in 1999 from the Cartoon Books site (now known as the Bonevillie website). 
On April 19th, 2021, Twitter user LostMediaMonsters published an image of the Warner Bros adaptation screenplay's front cover, which had previously been unknown to most of the public. Titled Bone: Return of the Dragon and written by Adam Kline and Mark Osborne, it had been produced sometime from late 2016 to early 2017.  While no specific details were given about the film, the user did state the full script could possibly be released.  On July 8th, 2022, Reddit user Kathmhen0 would publish higher quality versions of the Paramount version's concept art  alongside a mock-up poster for the aforementioned attempt, previously unseen.  Both posts' material were taken from a 2011 book known as Modern Masters Volume 25: Jeff Smith By Eric Nolen-Weathington.
To this day, It's still unknown how much (if any) animation was done for the Paramount version of the film if any more concept art exists for it, or any further material from the Warner Bros version is out there. Netflix's own attempt, albeit as an animated series, also has yet to have any of it's own production material released. With three cancelled adaptations of Bone, it's unlikely any of them will ever see the light of day.
- News article on Nick Movies buying the film rights to Bone from 1998. Retrieved 18 Jul '20
- An interview with Jeff Smith from 1999, detailing production on the film. Retrieved 18 Jul '20
- June 2000 article that mentions it would be traditional animation, the planned 2002 release, and the worrying fate of the movie before it was cancelled. Retrieved 6 Feb '21
- An interview with Jeff Smith from 2003 about the cancellation of the film. Retrieved 18 Jul '20
- News about the cancellation of the Nickelodeon adaptation of the film. Retrieved 18 Jul '20
- September 2000 interview with Comics2Film, discussing the aftermath of the film's demise and some other insights about the attempted feature, including the two pieces of concept art. Retrieved 6 Feb '21
- December 2002 IGN interview of Smith stating he had no current plans to make an animated version of Bone. Retrieved 6 Feb '21
- A news article about the Warner Bros. film's announcement. Retrieved 5 Feb '21
- 2009 article mentioning the plans for a CGI trilogy. Retrieved 6 Feb '21
- 2010 article stating the film will use motion capture. Retrieved 6 Feb '21
- 2009 Interview with producer Dan Lin, discussing the status of the film. Retrieved 5 Feb '21
- Smith reporting on early test footage for the Warner Bros. adaptation of Bone. Retrieved 18 Jul '20
- News article about the directors and producers assigned to Warner Bros. film adaptation of Bone. Retrieved 5 Feb '21
- News of the Warner Bros. Bone film adaptation getting new directors. Retrieved 18 Jul '20
- News of a Bone animated series coming to Netflix. Retrieved 18 Jul '20
- Announcement that Bone had been dropped by Netflix. Retrieved 3 Aug '22
- The proposed idea to simultaneously write the Warner Bros trilogy and planned directors. Retrieved 3 Aug '22
- Mention of the Warner Bros. adaptation at 20th Century Fox. Retrieved 3 Aug '22
- Animated Movies webpage featuring the Nick/Paramount Bone concept art. Retrieved 18 Aug '20
- Bone movie concept art from the Bone Vernal Equinox Slipcase Edition. Retrieved 18 Aug '20
- First sight of the screenplay from April 2017. Retrieved 19 Apr '21
- Bone: Return of the Dragon title screenplay page resurfacing after being unknown for years in April 2021. Retrieved 19 Apr '21
- HD uploads of the Paramount / Nickelodeon version's concept art. Retrieved 3 Aug '22
- Mock-up poster of the Paramount / Nickelodeon version. Retrieved 3 Aug '22