Even before Pearl Harbor, the skies around San Diego were always full of planes. They were prop planes, of course, and they made a lot of noise. You could tell the different roars, and after a while you didn't bother to look up.

But there was one we kids all watched for. It was sleek and slim, with twin fuselages like a catamaran, and it whipped past you without a sound, so smooth and fast you weren't quite sure if you'd really seen it. The roar came after.

Even the roar was smooth.

That was the P38.

The elegant Lightning was perhaps the only plane the American public actually loved just for the look of it.

The designer of that beautiful weapon was a man named Clarence L. (Kelly) Johnson, and he was in Washington yesterday with another legendary aircraft designer, Edward H. Heinemann, who designed the first plane to fly at Mach 2, twice the speed of sound. And some others. They were here for the opening of a new exhibit, "Designers for the Jet Age," at the Air and Space Museum.

"The P38 had only one seat," Johnson said, "so when I went on the test flight I had to sit piggyback on the pilot. Five hours at 25,000 feet. Its problem was its compressibility."

That is to say, when the plane went into a vertical dive approaching the speed of sound it tried to curl under and its tail juddered dangerously. Johnson redesigned the wing, and the P38 became a vital long-range fighter-escort in the Pacific theater of World War II.

Fighter pilots liked it despite a certain reputation as a jinx, because it could peel off equally well to right or left, a rare quality in those days when spinning propellers created a constant torque or twist to one side or the other. The P38's twin props turned in opposite directions, neutralizing the torque. It was the first plane to do this, Johnson said, since the Wright brothers' craft.

Johnson, 75, began drawing planes as a kid, decided at 12 that he would design real ones. He worked 50 years for Lockheed Corp., starting as a tool designer in 1933, and helped develop more than 50 planes, from the F104 Starfighter (the first operational jet to achieve Mach 2) to the famous U2 spy plane.

In 1943, working frantically against time and in deep secrecy, Kelly Johnson -- the nickname came from his green neckties -- designed and built the first American jet fighter, the F80 Shooting Star, using a British engine. He had only blueprints of the revolutionary deHavilland engine, and the hardware itself arrived only seven days before the airframe was completed. The engine cracked on the first test flight, so a new one had to be flown in from England. This time the watching engineers and executives saw what a jet could do.

As the Lockheed account has it, test pilot Milo Burcham "made one low pass across the field and went whistling up out of sight. Then the show began. Accustomed to the controls, Burcham came down from high altitude so fast that no one knew he was coming until he passed overhead and the roar hit the crowd . . ."

One of Johnson's favorites was the SR71 Blackbird, a '60s reconnaissance aircraft that flew literally faster than a bullet, at Mach 3 (Washington to Los Angeles in about an hour). Everything in the Blackbird had to be pioneered -- the titanium forgings that would hold at 800 degrees, the hydraulics and fuel systems, the escape system that had to work at 100,000 feet.

Today he is senior adviser to the "Skunk Works," Lockheed's advanced projects department, where, "if I can talk about it, it's obsolete."

He fully appreciates the time saved by computers today, especially in eliminating much of the trial and error in design details. But he still has his slide rule.

The 77-year-old Heinemann, like Johnson a Michigander displaced to California, has been a chief engineer at Douglas since 1936. Something of a prodigy, he quit high school at 17 and started building boats up to 60 feet long, studying on his own. Within a year Douglas snapped him up as a designer.

His creations include the SBD Dauntless dive bomber, the D5582 Skyrocket, the F4D Skyray and the A4 Skyhawk, which was in production for 25 years, the longest for any jet combat aircraft. He designed the AD1 Skyraider under a 24-hour deadline. He also designs boats, and yesterday he was handing out a brochure for the Super H3 patrol boat, "designed in the Heinemann tradition of cost-efficient military systems."

This is the catch in designing planes: It's one thing to draw a lovely air-smooth shape, but quite another to pack it efficiently with all its equipment.

"I never worked with contract engineers," Heinemann said. "I always had my own team. I was the head guy with all the responsibility. I ran the damn show. My people came in with an air conditioner that weighed 25 pounds when it was supposed to be 12 pounds, and I said, 'Out! Do it over!' They brought it back in at 10 pounds. Every pound you put on here you have to take away there."

Some say it was the carrier-based Dauntless that won the Battle of Midway and turned the tide of the Pacific war.

"I must have made 50 full-out dives in that thing in the rear seat behind Vance Breece," he said. "It was kind of exciting."

Every plane designer has a back-to-the-old-drawing-board story to tell. Recalled Heinemann, "Vance was also the pilot on the BT1 a Dauntless precursor the time Hugh Dryden, now of NASA, asked for a test dive. Vance came down and said, 'Goddam it, the bomb tail fell off!' and we were yelling and swearing at each other, and Dryden walked up and said politely, 'My goodness, are you still using that bomb? I tested that bomb in 1925.'

"I said, 'Well, we better build a proper bomb.' So I designed a basic shape for various weights and five different makes of planes, and it's still the international standard shape. I was an SOB in those days . . . but a good one."