Google announced the launch of Android P as a beta at I/O, but you don't have to go through the OTA beta program to get the latest developer preview. The system images are available for download, ready for flashing on your unlocked device. You still need one of the supported Google devices to install the system image for developer preview 2, though.
The system image gets you on the same preview version as the beta program, but it'll install on top of any software. You need an unlocked bootloader and ADB set up on your computer to flash the image. By default, this erases your phone. You can turn that off in the batch file, but that can lead to some weird bugs.
Google also announced the second developer preview would run on phones from other companies like OnePlus and Sony. However, the system images are only available for Google's phones. That means you need a Pixel, Pixel XL, Pixel 2, or Pixel 2 XL.
- Android Developers