Powder diffraction was invented by Peter Debye and Paul Scherrer but also independently by Albert Hull. All in 1917, during World War I. Working in Germany, Scherrer and Debye were fortunate to be Swiss and (then) Dutch citizens, respectively, as this exempted them from being drafted. Hull worked for a U.S. Company, General Electric, which was a defense contractor. Appropriately, Debye and Scherrer are given the scientific credit, as Hull did not publish his work promptly; he was discouraged from use company time for writing papers. Having also worked in U.S. companies, he has my sympathies, but I do note that Hull did go on to have several other important inventions, one leading to microwave ovens. Powder diffraction was used initially for the purpose of descriptive crystal-structure determination, for materials where single-crystal specimens for materials were not available. It was also initially deployed because the powder diffraction pattern provided a unique structural “fingerprint” for materials.
The name powder diffraction is really a misnomer, as it is used for much more than measurements on powders. A better name for the technique would be polycrystalline diffraction, as it can be used on any sort of bulk material that has a large number of unoriented small crystal components. However, with more than 100 years of tradition behind us, I think it is too late to lobby for a different name.