Is it possible for a Teensy 3.1 microcontroller to receive USB keyboard input?
Technically, it is possible. The board already has a Micro-AB port, and the MCU on it supports OTG. But all the examples now given are configuring it as a device. So you may need some coding to make it a host.



Update:

And, thanks to @PkP, he find their is a example for USB host in the MCU's datasheet, section 41.7.