Sony has surprise packed for its customer this year. Earlier they reported that atleast a dozen of their Xperia phone will get the Android 5.1 update and now it has released Android M Developer preview for atleast 16 Xperia devices.
The Android M Developer Preview is now available for Xperia Z3, Xperia Z3 Compact, Xperia Z3 Tablet Compact, Xperia Z2 Tablet, Xperia Z2, Xperia Z Ultra, Xperia Z1, Xperia Z1 Compact, Xperia Z, Xperia Tablet Z, Xperia ZL, Xperia E3, Xperia M2, Xperia T2 Ultra, Xperia L, Xperia S and Xperia T3. On the other hand, OEM like HTC has earlier confirmed Android M upgrade from only One M9 and One M9+ phone only. Comparatively this is a huge list from Sony.
Sony has also come up with a video show how Android M developer preview works on Xperia Z3 Compact phone. This AOSP version shows up 5.1.51 as the Android version.
You can download the Android M Developer Preview binaries from here and instructions on how to build AOSP M Developer preview goes here.