This feature is available in Enterprise and Ultimate editions
How can I see burndown for a complete project regardless of the sprint? I do not want to see any features or backlog items that are not tied to a sprint/iteration. Can that be accomplished using a report?
Yes. You can generate a Burndown report to see the planned backlog items in a release and to track progress against them. This is one benefit of defining a release in the project hierarchy. A release allows you to segment all those items that are actually planned to be delivered, allowing you to filter out all of the unplanned backlog items (or requirements) stored in the project.