Junkers Ju 87 with the Ju 87 A-0, the B-1s were returned discreetly to the Reich. The experience of the Spanish Civil War proved invaluable - air and ground crews perfected their skills, and equipment was evaluated under combat conditions. The Ju 87 had not been tested against numerous and well-coordinated fighter opposition; this lesson was learned later at great cost to the Stuka crews. All Stuka units were moved to Germany's eastern border in preparation for the invasion of Poland. On the morning of 15 August 1939, during a mass-formation dive-bombing demonstration for high-ranking commanders of the Luftwaffe at Neuhammer training grounds
Junkers marked on the J-9's landing gear wheel covers. The corrugated duralumin wing and fuselage "skin" introduced in the J-series became a trademark of Junkers aircraft built in the 1920s and '30s. Development continued during the course of World War I, including a growing (but troublesome) partnership with Fokker, as the "Junkers-Fokker Aktiengesellschaft". Several Junkers designs were licensed to Fokker during this period. The visual similarity of Junkers and Fokker aircraft during the next decade, especially after Reinhold Platz adapted some of the Junkers design concepts, but mostly crafted in wood for the Fokker designs' wing structures instead of the all-metal