with the assumption that there would be no escorts. This was due to a combination of factors. One was the belief that improved navigational technique would allow the night bomber force to attack point targets. Another was that immediately before the war, new and much larger aircraft like the Vickers Wellington were arriving that mounted heavy defensive armament. This convinced some in Bomber Command that daylight raids would not require escorts. The concept of daytime raids quickly ended after the disastrous air battle of the Heligoland Bight, in which a force of 22 Wellingtons lost 12 aircraft to German fighters as they attacked warships in harbour. For the rest of the war, the RAF was convinced night bombing was the only survivable strategy and this opinion was not seriously reconsidered. Both Fighter and Bomber command evolved doctrinal arguments against escorts, even after photoreconnaissance versions of the Spitfire were flying deep into Germany at ranges that would allow them to escort daytime bombing. During the preparations for the Big Week offensive, the USAAF asked the RAF to consider how to carry out daytime bombing with Spitfire escorts. Fighter Command claimed this was impossible, making several highly questionable calculations to "prove" this. This led to two Spitfires being shipped to Wright-Patterson where many new fuel tanks were added without major effects on performance. Their now excellent range was demonstrated by flying them back across the Atlantic to England. In spite of this demonstration, Fighter Command was slow to consider such adaptations and was not until well after Pointblank was concluded that final sign-off was given, at which point they saw no need for such conversions. In US Army Air Force service The U.S. Army Air Forces' precision strategic bombing campaign against German industries was only possible during the day. At first, this was not seen as an issue; the Forces' Boeing B-17 Flying Fortress and Consolidated B-24 Liberator bombers were the most heavily armed aircraft of the time. Close formations of them were planned, creating a crossfire of .50 caliber machine-guns that would fend off the enemy with no need for a fighter escort. Some officials claimed that escort fighters were wholly impractical. The service remained convinced of this strategy in spite of continued warnings from the RAF that this would not be the case. In the early stages of the US efforts, bombing runs often took place without escort fighters. German