Even though we live in a supposed "golden age" of flying, most flyers know that the amount of legroom you are given on plane is uncomfortable at best. This however, was not always the case. Although we do not have any specific measurements of earlier aircraft, during the 1950's, airliners claimed to offer between 34 and 36 inches (between two rows of seats).
However in the 1980's, this began to change. Airliner companies figured out that if you reduced leg room between each row of seats, you could fit in more buying customers. And so started the trend of cramped airliner seats.
Read more about it on the Traveler website.