B4A signs it when you compile (assuming you have setup your private sign key, which is a must).
You can find the APK here:
YourProjectFolder\Objects
Be sure to use the Clean Project feature in B4A (Tools menu) before compiling when preparing for distribution. I didn't initially realize the importance of doing this, but it can add unnecessary 'crap' to your APK, making it larger than it needs to be.