The first of the four planes [P4] to depart was American ­Airlines Flight 11, a Boeing 767-200ER. It was 159 feet and two inches long, with a sixteen-foot-six-inch-wide body that allowed for two aisles. The plane made daily flights between Boston and Los Angeles, and when it took off at 7:59 a.m. on the morning of the eleventh, it carried only 81 passengers in its 158 seats. Forty-seven minutes later, it crashed into the North Tower at 440 mph, carrying 9,717 gallons of jet fuel, 14,000 gallons under capacity.

United Flight 175, also a Boeing 767-200ER, was the second. Like American Airlines 11, it was scheduled to fly between ­Logan and LAX. When United 175 took off at 8:14 a.m., it was even lighter than the American flight: Only 56 out of 168 seats were occupied. When it crashed into the South Tower at 9:03 a.m., traveling 540 mph, it had 9,118 gallons of fuel in its tanks.

American Airlines Flight 77 was the third plane to take off that day, a Boeing 757-200. AA77 left Washington, D.C., at 8:20 a.m. bound for Los Angeles. It was two-thirds empty, with 58 passengers in its 176 seats, and its tanks were 4,000 gallons under its 11,500-gallon capacity. It crashed into the Pentagon at 9:37 a.m., ­flying 530 mph.

The fourth plane, United Airlines Flight 93, was also a 757-200. It was delayed for 42 minutes past its ­scheduled 8 a.m. departure from Newark bound for San Francisco. When it finally took off, it carried only 37 ­passengers—its capacity was 182—and it was loaded with a little over 7,000 gallons of fuel. It crashed at 560 mph into an empty field in Shanksville, Pennsylvania, at 10:03 a.m.

The two models—the 767 and the 757—were introduced within a year of one another in the early eighties, when Boeing was fighting lackluster sales, dwindling cash reserves, and a surging European rival, Airbus. The company marketed the planes to airlines as cost-savers, emphasizing their fuel efficiency and their modified cockpits, which allowed two pilots to do the work of three. Crews testing both aircraft gave them high marks for precise handling.

Related:

• Articles by Eric Benson
• Table of Contents: Sep 5, 2011 issue of New York | Subscribe!