In March 1942, General Arnold made an off-hand comment that the US could avoid the German U-boat menace by flying fighters to the UK (rather than packing them onto ships). President Roosevelt pressed the point, emphasizing his interest in the solution. Arnold was likely aware of the flying radius extension work being done on the P-38, which by this time had seen success with small drop tanks in the range of , the difference in capacity being the result of subcontractor production variation. Arnold ordered further tests with larger drop tanks in the range of ; the results were reported by Kelsey as providing the P-38 with a ferrying range. Because of available supply, the smaller drop tanks were used to fly Lightnings to the UK, the plan called Operation Bolero.