Photos: Jets were coming, the world knew. The huge and complicated piston engines powering 1950's airliners were nearing their limit of technical sophistication. But they still seemed a safe investment as the 1960's neared -- except to Pan Am's Juan Trippe. With single-minded purpose, he convinced America's biggest aircraft manufacturing rivals, Douglas and Boeing, to radically push the envelope of commercial air travel. Almost exactly 31 years after Pan Am's first 90-mile flight to Havana, the first Boeing B-707 jet was on its way from New York to Paris. There was no going back, not for Pan American or for the rest of the world.