For test purposes it is useful to bootstrap tails using virtual machines and files. Here is how it can be done with KVM.
$ wget 'https://tails-dl.urown.net/tails/stable/tails-amd64-3.3/tails-amd64-3.3.iso'
$ wget 'https://tails-dl.urown.net/tails/stable/tails-amd64-3.3/tails-amd64-3.3.iso.sig'
$ gpg --keyserver pgp.mit.edu --recv-key BA2C222F44AC00ED9899389398FEC6BC752A3DB6
$ gpg --verify tails-amd64-3.3.iso.sig tails-amd64-3.3.iso
$ qemu-img create -f raw tails-installed.img 8G
$ kvm -m 4096 -cdrom tails-amd64-3.3.iso -device usb-ehci \
-boot d \
-drive id=my_usb_disk,file=tails-installed.img,if=none,format=raw \
-device usb-storage,drive=my_usb_disk,removable=on \
-net nic -net user -display sdl
From the virtual machine console, install tails to the USB device (i.e. tails-installed.img). When the installation is complete Control-C KVM. Copy tails-installed.img to tails-backup.img in case you want to start over. Run tails with:
kvm -m 4096 -device usb-ehci \
-drive id=my_usb_disk,file=tails-installed.img,if=none,format=raw \
-device usb-storage,drive=my_usb_disk,removable=on \
-net nic -net user -display sdl