Perhaps one of the most iconic passenger planes ever built was the four-engined

The plane was a huge success for the company, ruling the skies for over 50 years.

has released several variations of the jumbo jet during its lifetime, with the last one being delivered to Atlas Air in January 2023.

One version of the plane that never made it off the drawing boards was the 747 Trijet.

In the 1960s and 1970s, the company toyed with building a three-engine commercial plane.

This would have seen an engine on either wing with another mounted on the tail, as normal in such planes.

The design would have been similar to that of the narrow-body 727, which was also a huge commercial success.

The plane manufacturer wanted a model that could compete with other Trijet airliners developed by their competitors.

These included the Lockheed L1011 and the McDonnell Douglas DC-10.

The 747 Trijet would have been able to accommodate more passengers, deliver a greater payload and have a longer range than its competitors.

However, the plane never saw the light of day due to two major problems.

To make the 747 Trijet structurally stable, engineers would have had to design a brand new wing.

This would be enormously expensive, with management eventually deciding that the cost could not be justified.

Secondly, the company wanted to maintain the new plane's existing handling characteristics so that pilot training would be easier and more cost-effective.

The company found it difficult to achieve this with two main engines on the wings and a third mounted in the tail.

The compromise solution was a shorter 747 with four conventional engines, the 747SP, with the suffix "Special Performance."

Entering service in 1976 with Pan Am, Boeing built 45 747SP aircraft.

The compromise solution was a shorter 747 with four conventional engines, the 747SP, with the suffix "Special Performance."

Entering service in 1976 with Pan Am, Boeing built 45 747SP aircraft.