Which bootloader works with the MKW31Z?
The KW41Z bootloader it's compatible with the KW31Z.
You can find it available on the SDK in the MCUXpresso web page.
Please notice that if you wish to use OTA it is a different bootloader, but you will find it available there.
Also, you can check the OpenSDA Serial and Debug Adapter|NXP for useful information.
If you have any doubt about it, please let me know.