Plan B is already built into a good B Plan.
When you are thinking about your venture, you are going to think about multiple scenarios, including very pessimistic ones and what you would do to mitigate the risks and the challenges in the journey. This should include what matrices you would use to track progress, and at stage you would take corrective action, including aborting the journey. It will be important to have an advisory board, or a real board, which will guide you through your decision making in good times as well as tough times.
Anyway, a business plan should take into account possibilities of failure, and hopefully the course correction that you might take in case the journey is not as you plan it to be.
In planning your business, including plan B, it is important to ensure that your assumptions are closer to reality. Wrong assumptions are more likely to kill a business than poor implementation.
A business plan is nothing but a plan for your business. While there are fancy templates, a business plan is nothing more than a story about
(a) what you are going to do i.e. concept
(b) how you are going to do it i.e. operations planning
(c) how you will make money i.e. business model
Of course, each has to be very detailed when thinking through a business plan. The excel sheet is nothing more than a summary of costs and revenues associated with your story. The power-point version of this story includes context i.e. why is there a need for this concept, who is your competition, the team who is doing this, etc.
Also remember that a business plan is a ‘process’ and not a product. And hence, while it provides direction, the route has to be constantly adjusted according to how the venture progresses. This process of evolving the B Plan is in a way also about working on Plan B.