The Legion's bomber group temporarily gave up daylight bombing after the failed Fascist offensive. Sperrle's airmen switched to attacking choke-points around Madrid. Operations were complicated by the Sierra de Guadarrama and Sierra de Gredos mountain range. Pilots suffered from circulation problems in non-pressurised cockpits while flying close to the top of the peaks, but only one aircraft and five men were lost to the natural barriers. Influenza and pneumonia outbreaks due to poor living conditions continued to undermine morale. One of the main problems was the inadequate fighter aircraft. The Heinkel He 51 was not up to the job, Sperrle requested Berlin send modern aircraft. In December three Messerschmitt Bf 109 fighters were in Spain, only to return the following February, followed by a single Heinkel He 112 prototype, and Junkers Ju 87 dive-bomber. Richthofen travelled to Berlin to secure more of these types in agreement with chief of staff Albert Kesselring, Ernst Udet, and Milch. Sperrle's command began receiving the Dornier Do 17, He 111, Ju 86 bombers and Bf 109 fighters in January 1937. On 1 April 1937 Sperrle was promoted to Generalleutnant (Major General). The Nationalists turned to destroying the Republican army enclave along the Bay of Biscay in the summer, 1937. Sperrle moved his headquarters to Vitoria to lead his small force of 62 aircraft. The Biscay Campaign began on 31 March with the Bombing of Durango. Spanish nationalists reported Republican army movements in the town, but before the bombers arrived they withdrew, leaving the bombers to devastate the town and kill 250 civilians. Sperrle protested to the Nationalists about the "waste of resources", but his own authorised attacks on the town's arms-factory were inaccurate and did just as much to damage the Legion's image. Thereafter Sperrle supported the battle for Otxandio. The Nationalists refused to cooperate in an encirclement operation allowing the defenders to retreat toward Guernica. In a farcical moment Sperrle was arrested on a visit to the front by Nationalist forces as a suspected spy. Sperrle's staff had no file information on Guernica. Richthofen earmarked the town's Rentaria Bridge for attack, but he seems to have been more interested in Guerricaiz six elms (10 km) south east of Guernica. Richthofen referred to this town as "Guernicaiz" in his diary. The Republican left flank had to retreat through Guernica and the air staff earmarked it for bombing. Richthofen's diary entry, dated 26 April 1937,