But very few people know the whole story of Jumbo, and a Google search will probably not find the story.

General Groves proposed that a scaled-down test be conducted in a containment sphere. That was where the whole idea came from. He was concerned about gambling on a billion dolars worth of plutonium. The idea was to intentionally make a small bomb that would not vaporize Jumbo and then recover the plutonium for a real bomb. But Oppenheimer could not guarantee that the small scale test could actually be contained. He argued that the small bomb might be bigger than expected and they would still end up losing the plutonium.

It was not proposed as backup in case the Trinity test failed. That was Oppenheimer's alternate proposal, but he only did that get Groves calmed down enough to permit the plans for a test. Nobody was ever really happy with the idea for Jumbo being part of Trinity.