Here are the general steps involved in developing a multi-hotel booking engine (B2B) for hotel chains:

1. Choose the technology stack: You can choose from a range of programming languages, frameworks, and databases. Common choices for web development include PHP, Python, Ruby on Rails, Node.js, and Java. You also need to select a database, such as MySQL, PostgreSQL, or MongoDB, to store booking information.

2. Obtain API access: You need to sign up for an API and obtain access credentials. The API should provide real-time access to inventory and pricing data from all hotels in the chain.

3. Build the booking engine: The booking engine should have a user-friendly interface that allows travel agents or tour operators to search for hotels based on various criteria such as location, amenities, and price. The engine should display hotel availability, pricing, and other information in real-time.

4. Integrate payment gateway: The booking engine should integrate with a payment gateway to process payments securely. You can use popular payment gateways like Stripe, PayPal, or Braintree.

5. Add additional features: You can add additional features like booking management, reporting and analytics, customer support, and marketing tools.

6. Test and deploy: Before deploying the booking engine, you should test it thoroughly to ensure that it works as expected. You can use automated testing tools and manual testing to identify and fix any bugs or issues.

7. Launch and maintain: Once the booking engine is deployed, you can launch it and start promoting it to travel agents and tour operators. You also need to maintain the booking engine by updating it regularly, fixing any bugs or issues, and adding new features based on customer feedback.

Developing a multi-hotel booking engine (B2B) for hotel chains can be complex and time-consuming. You may consider partnering with a software development company that specializes in travel technology to help you build a custom booking engine that meets your specific requirements.