 By February 1945 the seventeen aircraft of the second increment were themselves in need of upgrades , particularly those of the 216th Army Air Force Base Unit . Four of the planes assigned to the 393d Bombardment Squadron ( now part of the 509th Composite Group ) were transferred to the 216th to meet an increase in its testing tempo . Rather than attempt to modify the existing aircraft a few at a time , a decision was made to start a new production series . The first five of this third increment , known as Project 98228 @-@ S , also went to the test unit . The order totaled an additional 28 aircraft , with delivery of 15 designated combat models for the 393d beginning in April . The final eight were not delivered until after the two atomic bomb missions in August . Two were given to the 216th while the remaining six were assigned to the 509th at Wendover as replacements for any bombers lost while operating from Tinian . Ironically , no bombers were lost during operations from Tinian , but five of these six were lost in crashes over the next few years . 
