What should you do if the internal engine fire doesn't stop after windmilling for 5 minutes?

Continuing to windmill and injecting an extinguishing agent into the intake is a critical procedure in dealing with an internal engine fire that persists for an extended period. By windmilling, the engine is allowed to spool down naturally, which can help reduce the fire's intensity, and introducing an extinguishing agent directly into the intake can effectively suppress the flames and mitigate further damage. This action is designed to maximize safety and control the situation until the fire is fully extinguished or more definitive action can be taken.

The other options do not align with the most effective emergency response. Ceasing all activities, waiting for further instructions, or notifying maintenance does not provide immediate remedial action to suppress the fire. Shutting down the engine may prevent further damage but also could allow the fire to spread without active measures being taken. Thus, option B is the most appropriate procedure to manage an ongoing internal engine fire effectively.