A large part of how he did so without the British or French catching on was by creating the Metallurgische Forschungsgesellschaft. It was a company that made arms purchases with other large German companies in exchange for promissory notes with a five-year term holding 4% interest (MEFO bills). Those bills just so happened to be guaranteed by the German government. Also, the company didn’t actually do anything. Because it was a company rather than a government agency, it didn’t show up as arms purchases in government budgets which foreign countries would watch. But because the notes had the German government’s backing, everyone knew who really would pay their value, even if everyone pretended it was the “company.”

Of course, this had the side effect of giving Germany an absolutely enormous debt to pay when those bills’s times ran out circa 1939 (more debt than the official government debt, for reference). Hitler and the German leadership staved this off by paying some with looted Austrian and Czech treasuries and extending the rest “by a little while” an indefinite number of times. Then WWII came and destroyed the Nazi leadership, so many who had MEFO bills never got their returns.