Browsing through the excellent sight Cover Browser, I came across their collection of National Geographic covers. I thought it was way cool to see how much, and also how little, the design has changed over time. Watch as the printing technologies improve, colors increase in vibrancy, pictures are added and then slowly sharpened, the typeface of the magazine title changes, the 'contents' text takes up less and less space (to be dominated by the gorgeous photography), and an ornate border introduced around 1910 is slowly eliminated over time.
And for further National Geographic design, Alex Charchar of Retinart does an excellent break down from the graphic design viewpoint of not just the cover but also the content and page design within the magazine in The Timeless Beauty of National Geographic. But I'm getting ahead of myself, so, starting with the first ever issue from 1888, check out National Geographic Magazine's 125 year cover evolution as pictorially summarized by Everywhere Art: