The DC Extended Universe's version of the Justice League - as assembled by Ben Affleck's Batman and Gal Gadot's Wonder Woman - will assemble under Man of Steel and Batman V Superman director Zack Snyder's watch in 2017. Where the first team-up for Earth's Mightiest Heroes in the Marvel Cinematic Universe was preceded by multiple solo Avenger films, the DCEU is taking a different approach with the members of "the League". Diana Prince's backstory will be revealed during her solo movie in 2017, but the solo vehicles for Ezra Miller's Barry Allen and Jason Momoa as Arthur Curry won't start filming until after Justice League is done, next year (with the Ray Fisher-starring Cyborg not scheduled to arrive until 2020).
The Flash has Rick Famuyiwa (Dope) onboard as director, with Miller headlining and Dope costar Kiersey Clemons lined up to play Barry's loyal ally, Iris West. Although the film recently lost its official March 2018 theatrical release, the fact that additional casting is actively underway for Famuyiwa's DC superhero movie suggests it's still moving forward to make a 2018 release date. And now, the latest update on The Flash has corroborated as much.
OLV is reporting that The Flash is gearing up to begin principal photography in London by January 2017 - further suggesting that a theatrical release the following year is in the cards for Famuyiwa's movie. Justice League is now in the midst of production at Warner Bros. Studios, Leavesden, so The Flash ought to be the next DCEU installment to film on that studio space after Snyder's movie wraps up.
There's an October 2018 release date staked out for an "untitled" DCEU installment that The Flash could ultimately wind up taking. There are other possibilities too; for example, if Aquaman director James Wan wants more time to refine the film's script, The Flash could take the former's July 2018 date and Aquaman can move to the October date, instead. Either way, we can probably expect an update on The Flash's release date situation in the foreseeable future (maybe even before the cameras start rolling on the movie).
On a related note: there are signs that WB/DC have learned a lesson, following what was reportedly a difficult and rushed development process on David Ayer's newly-released DCEU addition, Suicide Squad. Multiple sources have now reported that studio executives tried to make significant changes to Suicide Squad after Ayer had filmed his vision for the movie. WB/DC now appears to be following Marvel Studios' lead and working out any such creative differences before a single frame of footage is filmed on its upcoming DCEU movies (as Marvel is known for doing with its MCU films).
For example, Aquaman's script continues to be further refined, until it's to the approval of everyone involved. Similarly, The Flash originally had Seth Grahame-Smith (screenwriter of Abraham Lincoln: Vampire Hunter and WB's upcoming animated LEGO Batman Movie) lined up to direct, but Grahame-Smith was replaced by Famuyiwa in June 2016: more than six months before filming is slated to begin. It was also reported at that time that Famuyiwa was hired because he was a "very compatible" fit to direct Grahame-Smith's Flash script draft, based on the treatment by Phil Lord and Chris Miller (The LEGO Movie).
In other words: there shouldn't be a repeat of the Suicide Squad situation, where Ayer reportedly had to put together his script draft in only six weeks. That's all the more encouraging to hear, given the potential for greatness in a Flash movie that pairs a skilled and energetic storyteller in Famuyiwa behind the camera with up-and-coming talents such as Miller and Clemons onscreen.
Suicide Squad is in theaters now. Wonder Woman opens on June 2, 2017; Justice League on November 17, 2017; Aquaman on July 27, 2018; an untitled DC Film on October 5, 2018; Shazam on April 5, 2019; Justice League 2 on June 14, 2019; an untitled DC film on November 1, 2019; Cyborg on April 3, 2020; and Green Lantern Corps on July 24, 2020. The Flash, the Batman solo movie and Man of Steel 2 are currently without release dates.