The research proposal is definitely a formal piece of writing. In general, it will address a few questions:

• What is your topic?
• What is your preliminary thesis?
• Why is it relevant, and how does it differ from previous scholarship?
• What sources will you use?

Some research proposals may also want a blueprint--a quick run-down of what you will be discussing in each chapter of your thesis.

You might want to consider /r/askacademia, which is a forum that frequently gets questions as to this subject matter, and better suited than this one, as the question is technically not a historical question.