Aviation inventors have often looked into the future for advances in design long before they were practical. Retractable landing gear is one such example. Leonardo da Vinci first conceived of this feature in the sixteenth century and the first patent was granted in the 1870s. J.V. Martin's Kitten was the first aircraft with retractable gear to be offered to the U.S. Army Air Service in 1918, but it did not come into universal use until the late 1930s. The concept of wing sweep has had a similar career.
