Did you make a bootable Dell boot and repair flash drive? If so, boot in UEFI mode and you can use it to restore the system to the way it shipped.
 I hav just bought inspiron 5521...I formatted entire HDD using ubuntu and when i tried to reinstall my win 8 back it didnt recognized bootable win 8 usb....the only OS it detected was win 7 professional 32 bit...i m screwed... i want my win 8 back and i didnt made any sort of backup or repair drive...so provide with.a concrete solution....i just hav bootable win 8 usb n tl how to install using IT ONLY win 8 on my laptop...sorry i m just adding threads bt i cant reply cozz of some problem with my scripts n plugins...Can u please provide solutions using emai n chat too using email...mine email is ....n as some experts said to install same window that existed earlier bt i dont hav that... 
Installing win 8 on inspiron 5521