The system should come with Windows 8.1 if just purchased. You can request a Reinstallation DVD after the system has arrived. If you reside in the US or CA you can fill in the online media request form:
If you don't reside in the US or CA, Dell haven't extended the courtesy of the media request form to other regions so you must call technical support. They seem often reluctant to send out recovery media so state that you have made your system non-bootable and that you cannot use Dell Backup And Recovery and will return the system if they try and persuade you not to procure a Reinstallation DVD.
Unfortunately you may only procure media once the system has arrived and you have the service tag.
The procedure regarding Reinstallation DVDs and the Windows 8.1 retail .iso is highly inefficient I have conveyed statistics to Dell here; users want to procure the DVD with the system and/or download it:
Please add your vote to the idea.
P.S. if you use the retail Windows 8.1 .iso, the .iso will automatically input your product key and activate thus you can use the retail license on a previous system and clean install Windows 8.1 on your XPS 8700. If going down that route and Dell send you a system with a Windows 8.0 product key the following steps will need to be followed:
See my Windows Reinstallation Guide/A Clean Install of Windows 8.1
for detailed installation instructions.