Android P is now available as a Developer Preview! It’s currently available to the Google Pixel, Google Pixel XL, Google Pixel 2, and Google Pixel 2 XL at this time.
This is the first Android P build and is labeled as an “alpha” release. It’s for developers here, rather than being used as a daily driver.
Remember this is not a beta, so this isn’t available in the Android Beta Program. Follow the source links for install instructions and file downloads.
Source: Instructions and factory images