Not sure I know how to dd it to a "file"
dd if=/dev/sdb of=~/usb.img
This command will create a file that contains an image of the USB drive.
Install Testdisk on your Linux system, a tool specifically designed for cases like yours. It will scan the USB device and re-create the partition table based on its findings. If this doesn't work (which it should, if only the partition table was wiped), it also brings the Photorec program, which is used to do file based recovery.