Early war planes were very light and used small engines with top speeds of less than 100 mph. On many designs the engine was in the rear and pushed the plane through the air. The demands of wartime, each side trying to outdo the other’s technological advances, created rapid improvements in aircraft design. Changes might occur within weeks; in the decades following the war, such changes would take years. By war’s end small, single-engine planes had been joined by multi-engine bombers such as the Giant, which Germany used to bomb British cities. Zeppelins were also used for reconnaissance and for bombing over land and sea. Tethered barrage balloons carried observers high above the front to watch enemy troop movements—and attracted the attention of the enemy’s airborne fighters.