Boeing RB-17 Flying Fortress
From the end of the 1940s a number of redundant B-17 Flying Fortress bombers were converted to RB-17 reconnaissance aircraft and began to probe the borders of the Soviet Union, who had by then begun to emerge as a potential threat to the Western Powers. One of the main units involved in this activity was the 55th Strategic Reconnaissance Wing based at Topeka, Kansas. Little was known about the air defence capability of the Soviet Union at this time and the most effective way of determining their capability was to probe the borders and see whether they would respond. Gradually the RB-17s and other aircraft mapped the perimeter of the Soviet Air Defences from the Baltic to the Sea of Okhotsk, north of Japan.
However, the most significant area was Russia's vast northern border, directly over the North Pole from North America. Russia was heavily committed to their massive land army stationed in Europe to counter a perceived ground threat from the Western Powers. At the same time Russia either did not believe the new USAF was a credible threat or they simply could not afford to extend their radar coverage along the entire length of their border, because the border reconnaissance flights soon discovered an almost total lack of radar coverage along a large part of northern Siberia.
In 1947 an RB-17 from Greenland and an RB-29 from Alaska flew 'Sitting Duck' combined penetration missions along the polar icecap several hundred miles apart. This mission, along with many others, found that west of the Bering Strait there was virtually no radar coverage. As a result of these missions, USAF war plans were drawn up which directed a massive bomber attack to hit Russia from this direction, flying on to land in the Middle East or Africa, or more likely bailing out out as the aircraft ran out of fuel. Gradually, during the late 1950's, the Soviets began filling in the gaps in their radar coverage over northern Siberia with P-14 'Tall King' radar's, but large gaps on the outer perimeter between Alaska and Murmansk were still wide open for many years to come.