Wargaming.net's World of Warplanes way back in the summer of 2011. Since then, the company has pumped out quite a few screenshots, several interviews, and press releases regarding its aviation-themed follow-up to World of Tanks.
The game is still in testing as of today, and Wargaming.net is kicking off what it calls a global alpha designed to bring in new testers from North America, Europe, and Russia. Each region has its own website and alpha sign-up process, so head to your respective portal right away if you're itching to be a test pilot.
The current client offers access to 20-plus aircraft and two battle arenas; Wargaming.net says that "the amount of content will be growing steadily" as the test phases progress.
[Source: Wargaming.net press release]