USAF Operations in the Korean Conflict 25 June - 1 November 1950 :
Interdiction - Railroads & Bridges
The FEAF Bomber Command dealt expeditiously with communications choke points assigned to it, as is indicated by figure 7.
On 13 July the Wŏnsan marshaling yards had been attacked by the newly-arrived 22nd and 92nd Groups on their shake-down mission. Sŏul marshaling yards were hit on MacArthur's special order on 16 July.
On 7 August the two groups, joined by planes of the 98th Group which had left the U.S. only five days earlier, plastered the marshaling yards at P'yŏngyang. Hamhung marshaling yards were attacked by newly arriving 307th Group B-29's next day. Smaller forces of B-29's also attacked the marshaling yards at
during August, while additional effort was placed against rail repair facilities at Wŏnsan and P'yŏngyang.
FEAF recognized that attacks against marshaling yards were chiefly valuable for the destruction of rolling stock concentrated there. Smaller missions attacked the key bridges assigned for Bomber Command destruction, and with a little experience the B-29 crews became exceptionally proficient in such work .
The most successful bombing tactic and the one generally used against bridges by medium bomber crews was an individual aircraft attack at an angle of 40 degrees, each plane releasing a string of four bombs on its run. Two groups adopted a procedure of dropping one bomb in the first run to obtain correct ballistic data and establish target altitude. In several instances the bridge so attacked was destroyed by this one bomb, but other bridges required many direct hits. Bomber Command, in fact, computed that 13.3 runs were required to destroy an average bridge, this number including multiple runs against a bridge by the same aircraft. For bridge attacks, Bomber Command generally used 500-pound GP bombs, admittedly not always the best ordnance, but crews often had to do their own loading and the command had to be prepared for last-minute changes in mission. In addition, larger tonnages of these bombs could be racked up in the B-29's than the heavier types.
The 500-pound bomb, dropped with a minimum intervalometer setting, was found satisfactory for flat concrete spans, but 1,000-pound or larger bombs were required for steel bridges. The degree of proficiency obtained in such attacks was shown in mission accomplishments: by 30 August Bomber Command had about completed work on the 44 key bridge targets assigned, and when on 4 September FEAF listed 56 more, Bomber Command destroyed 12 of them in three days
United States Air Force Operations in the Korean Conflict 25 June - 1 November 1950 was prepared 1 July 1952 as USAF Historical Study No. 71.
The study was researched by Air Force Historian Dr. Albert Simpson and Dr. Robert Futrell. Dr. Futrell wrote the 115+ page report along with several follow-up studies leading to the comprehensive "The United States Air Force in Korea." originally published in 1961 and reprinted in 1996 (ISBN 912799-71-4).