From XMind 2012, XMind supports saving the editing history of an .xmind file. You can check these histories in the Editing History view, which can be opened from menu Window -- Editing History.
Every time when we click Ctrl+S or the save button on the toolbar, XMind will save a revision in the Editing History view with revision no., the saving date and the specific time.
In this view, we can review, delete or revert to any revision.
Tip: The Editing history feature will save every revision with screenshot. So it may increase the file size, esp. the frequently modified file.