OP - That’s got to feel terrible. And it’s not very good management on your boss’s part. I suggest in the future you spend a bit more effort on communicating with your boss when you are assigned an important project. I’m not saying you seek his micromanagement, just keep him informed about progress. That works two ways: First it lets Boss know you are actively working on the project making it less likely they will also assign the project to someone else. Secondly it gives Boss the chance to provide input if they think you are straying too far from their intent. I learned that lesson a long time ago. It’s a balancing act, not appearing too needy of continuous direction while keeping boss informed and being open to their suggestions. A more effective Boss will communicate to you what they expect, but not all are as effective as they should be.

You still got paid, right?

A “huge” project that only took 3 days?