This question is a year old but I am going to answer it incase someone else runs into this same issue.
I just bought a mid 2011 27" refurb iMac from IBM and they scrubbed the HD completely. This eliminated pretty much every option there was for installing an OS on the Mac.
The problem is that when these models shipped they had either Lion (10.7) or at the earliest Snow Leopard 10.6.6. They are not backwards compatible with the Snow Leopard retail DVDs Apple offers, which is OS X 10.6.3. These iMacs originally shipped with installion DVDs but Apple would not send them to me.
Here's what I had to do:
Download/or otherwise obtain an OS X 10.6.7.iso file from wherever you might find something like that. It will be an disk image for a DVD not intended for this model of iMac, and it will not work.
I bought the retail DVDs (10.6.3) from Apple, and they did not work. I got a RAM Error message (three tones) at boot.
If you have both you can splice the installer package files from the retail DVDs (which work with any Mac) with the installer package files from the 10.6.7 disk image and create a "universal" OS X 10.6.7 installer disk image. This is not fancy it's a simple matter of copy and paste. Detailed instructions for this process can be found here.
After I had the modified .iso file I converted it into a .dmg using this terminal code. Again, very simple. Nothing to be afraid of.
From there I used "Carbon Copy Cloner" and Cloned the .dmg file I created through this process unto a thumb drive (needs to be at least 8gb). Worked like a charm. Just pup it in a usb port and turn on the iMac.
You can also (possibly) use Disk Utility which is a built in utility and "restore" the image to your thumb drive, but I wasn't having any luck with my Disk Utility boot disks and so I decided to try another utility. My feeling is that it likely would have worked this way if i had used DU.
Make sure your thumb is formatted for GUID. Just a word of caution. I intend to upgrade to Yosemite. The install of Snow Leopard works, but I don't fully trust it and my Airport card is non functional. Certain things went wrong in the installation process obviously, but it's good enough to boot, and then upgrade, or maybe try to install a fresh unaltered version of Snow Leopard if that's your desire.
I hope this helps someone out. I've tried dozens of different ways to just get this iMac to boot an OS and this is finally the one that worked. Apple Support was zero help. Good luck!
- I've since updated Snow Leopard to 10.6.8 and airport is now working. Everything is very smooth and functional. No problems.