TL;DR: Follow the instructions exactly HERE AND use a USB 2 Flash drive
I haven't had to install Windows on a Mac in a while but recently ran into all kinds of issues attempting to simply install Windows 7 64-bit on a 2014 MacBook Air. First of all, if you just fire up boot camp and follow the prompts to get things installed, you're gonna have a bad time. This will no longer work on newer macs with USB 3.0 ports for Windows 7.
You need to follow the instructions exactly as listed on Apple's support page for Boot Camp to get your bootable Windows 7 64-bit USB flash drive ready. The important part is to pay attention to STEPS 6-11 under "Perform a new install of Windows 7":
"6. Select only the options to create a Windows install disk and to download the latest Windows support software from Apple. Then click Continue."
There is an important step you have to manually complete before you can select the option to install Windows in the Boot Camp Assistant. You have to look up and download the correct Boot camp version for your Mac, then manually copy the folders from that download to the bootable Windows 7 flash drive you create. Only after completing these steps can you then reopen Boot Camp Assistant and select only the option to "Install Windows… or later version."
I followed these instructions EXACTLY on a 2014 13" MBA and still got errors during the Windows 7 install that were driver related. Error during Windows 7 install:
"A required CD/DVD drive device driver is missing. If you have a driver floppy disk, CD, DVD, or USB flash drive, please insert it now."
I FINALLY got it to work by starting completely over and using a USB 2 flash drive (NOT USB 3).