Here are what I think are the key ingredients of a successful history Ph.D. proposal:
1. A
statement of the topic and why it is important and interesting.
2. A
summary of the existing historiography.
3.
Some reflection on the deficiencies/limitations of the historiography.
4. A description of the sources that you intend to use, and how they will enable you to
overcome those limitations.
Incidentally, now is the time of year that you need to start exploring funding opportunities too.