The USB armory from Inverse Path is an open source hardware design, implementing a flash drive sized computer. Kali Linux fits on a micro SD card for it.

Kali on USB armory - User Instructions

If all you want to do is install Kali on your USB armory, follow these instructions:

1. Get a nice fast 8 GB micro SD card.
2. Download the Kali USB armory image from our downloads area.
3. Use the dd utility to image this file to your microSD device. In our example, we use a microSD which is located at /dev/sdb. Change this as needed.

xzcat kali-linux-$version-usbarmory.img.xz | dd of=/dev/sdb bs=4M

This process can take awhile depending on your device speed and image size.

Once the dd operation is complete, boot up a computer with the USB armory plugged in. Log in to Kali, that's it, you're done!

Kali on USB armory - Developer Instructions

If you are a developer and want to tinker with the Kali USB armory image, including changing the kernel configuration and generally being adventurous, check out the kali-arm-build-scripts repository on GitHub, and follow the README.md file's instructions. The script to use is usbarmory.sh