Antimicrobial peptides (AMPs) are being developed as potent alternative treatments to conventional antibiotics which are unlikely to induce bacterial resistance. They can be designed and modified to possess several druggable properties. We report herein a novel hybrid peptide of modified aurein (A3) and cathelicidin (P7), or A3P7, by a flipping technique. It exhibited potent antibacterial activity against both Gram-negative and -positive pathogenic bacteria but had moderate hemolytic activity. To reduce the sequence length and toxicity, C-terminal truncation was serially performed and eight truncated derivatives (AP12-AP19) were obtained. They had significantly less hemolytic activity while preserving antibacterial activity. Secondary structures of the candidate peptides in environments simulating bacterial membranes (30 mM SDS and 50% TFE), determined by CD spectroscopy, showed α-helical structures consistent with predicted in silico 3D structural models. Among the peptides, AP19 demonstrated the best combination of broad-spectrum antibacterial activity (including toward Acinetobacter baumannii) and minimal hemolytic and cytotoxic activities. A D-form peptide (D-AP19), in which all L-enantiomers were substituted with the D-enantiomers, maintained antibacterial activity in the presence of pepsin, trypsin, proteinase K and human plasma. Both isomers exhibited potent antibacterial activity against multi-drug (MDR) and extensively-drug resistant (XDR) clinical isolates of A. baumannii comparable to the traditional antibiotic, meropenem. D-AP19 displayed rapid killing via membrane disruption and leakage of intracellular contents. Additionally, it showed a low tendency to induce bacterial resistance. Our work suggested that D-AP19 could be further optimized and developed as a novel compound potentially for fighting against MDR or XDR A. baumannii.