GPA Group Ltd., an Irish aircraft-leasing company, announced Tuesday it was ordering 308 planes, the largest single purchase of aircraft in aviation history. The $16.8 billion order will be shared by the American companies Boeing and McDonnell-Douglas, and Airbus, a European consortium, company officials said. A similar announcement was made in London Tuesday. The biggest previous aviation order - worth $10.5 billion - was placed by Delta Air Lines last September for 225 orders and options on Boeing and McDonnell-Douglas airplanes.