The twenty-one years between the end of World War I (November, 1918) and the start of World War II (September, 1939) saw an unprecedented advance in aviation technology. Aircraft design changes in fighters changed drastically from wire struts and fabric skin biplanes to all metal single wing marvels with speeds approaching 3 times their predecessors.
Firepower of these modern machines advanced even more so. The age of one or two machineguns (7.92mm or .30 caliber) firing through the propeller arc, timed with a rudimentary interrupter system or with steel plates on wooden propellers gave way to heavy machineguns (13mm or .52 caliber) and automatic cannon (20mm and 30mm) rounds with high explosive warheads.
The first bombers carried a crew of 2 to 4 men, flew at 90 miles per hour, and carried maybe 1,000 pounds of bombs. Two gunners provided protection from enemy fighters, firing from open cockpits with handheld guns mounted on a scarf ring. Later versions evolved to massive aircraft with crews of 10 men with multiple gun turrets protecting the bomber from all directions while carrying up to 8,000 pounds of high explosives.
Late in the war appeared the first jet fighters, flying at speeds approaching the speed of sound. Ballistic missile systems as well as the first cruise missiles made their appearance and wrought great destruction. They were the harbingers of today's ICBM and stealth fighters.